Implemented Figure.grdcontour that uses grdcontour gmt module

Plots grids to contours, with its own set of distinct inputs from `pscontour`

Test and Initial Documentation is complete

Author: akshmakov

doc/api/index.rst

@@ -24,6 +24,7 @@ Plotting data and laying out the map:
     Figure.coast
     Figure.plot
     Figure.contour
+    Figure.grdcontour
     Figure.grdimage
     Figure.logo
 

gmt/base_plotting.py

@@ -125,8 +125,51 @@ def coast(self, **kwargs):
         with Session() as lib:
             lib.call_module("coast", build_arg_string(kwargs))
 
+
+    @fmt_docstring
+    @use_alias(
+        R="region",
+        J="projection",
+        A="annotation_interval",
+        C="contour_interval",
+        B="frame",
+        G="label_placement",
+        L="limit",
+        W="pen",
+    )
+    @kwargs_to_strings(R="sequence")
+    def grdcontour(self, grid, **kwargs):
+        """
+        Convert grids or images to contours and plot them on maps
+
+        Takes a grid file name or an xarray.DataArray object as input.
+
+        {gmt_module_docs}
+
+        {aliases}
+
+        Parameters
+        ----------
+        grid : str or xarray.DataArray
+            The file name of the input grid or the grid loaded as a DataArray.
+
+        """
+        kwargs = self._preprocess(**kwargs)
+        kind = data_kind(grid, None, None)
+        with Session() as lib:
+            if kind == "file":
+                file_context = dummy_context(grid)
+            elif kind == "grid":
+                file_context = lib.virtualfile_from_grid(grid)
+            else:
+                raise GMTInvalidInput("Unrecognized data type: {}".format(type(grid)))
+            with file_context as fname:
+                arg_str = " ".join([fname, build_arg_string(kwargs)])
+                lib.call_module("grdcontour", arg_str)
+
+
     @fmt_docstring
-    @use_alias(R="region", J="projection", B="frame", I="shading", C="cmap")
+    @use_alias(R="region", J="projection", W="pen", B="frame", I="shading", C="cmap")
     @kwargs_to_strings(R="sequence")
     def grdimage(self, grid, **kwargs):
         """

gmt/tests/baseline/test_grdcontour.png

@@ -0,0 +1,70 @@
+"""
+Test Figure.grdcontour
+"""
+import pytest
+import numpy as np
+
+from .. import Figure
+from ..exceptions import GMTInvalidInput
+from ..datasets import load_earth_relief
+
+@pytest.mark.mpl_image_compare
+def test_grdcontour():
+    """Plot a contour image using an xarray grid
+    with fixed contour interval
+    """    
+    grid = load_earth_relief()
+    fig = Figure()
+    fig.grdcontour(grid,
+                   contour_interval="1000",
+                   projection="W0/6i")
+    return fig
+
+@pytest.mark.mpl_image_compare
+def test_grdcontour_labels():
+    """Plot a contour image using a xarray grid
+    with contour labels and alternate colors
+    """
+    grid = load_earth_relief()
+    fig = Figure()
+    fig.grdcontour(grid,
+                   contour_interval="1000",
+                   annotation_interval="5000",
+                   projection="W0/6i",
+                   pen=["a1p,red","c0.5p,black"],
+                   label_placement="d3i",
+                   )
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_grdcontour_slice():
+    "Plot an contour image using an xarray grid that has been sliced"
+    grid = load_earth_relief().sel(lat=slice(-30, 30))
+    fig = Figure()
+    fig.grdcontour(grid,
+                   contour_interval="1000",
+                   projection="M6i")
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_grdcontour_file():
+    "Plot a contour image using grid file input"
+    fig = Figure()
+    fig.grdcontour(
+        "@earth_relief_60m",
+        contour_interval="1000",
+        limit="0",
+        pen="0.5p,black",
+        region=[-180, 180, -70, 70],
+        projection="M10i",
+    )
+    return fig
+
+
+def test_grdcontour_fails():
+    "Should fail for unrecognized input"
+    fig = Figure()
+    with pytest.raises(GMTInvalidInput):
+        fig.grdcontour(np.arange(20).reshape((4, 5)))