Add an example cube netCDF file

[seisman]

We need a cube netCDF file for testing when wrapping the GMT_CUBE data structure in PyGMT (GenericMappingTools/pygmt#3150). I found the cube file cube.nc in the GMT repository: https://github.com/GenericMappingTools/gmt/tree/master/test/grdinterpolate.

Here is the cube information:

gmt grdinfo cube.nc
cube.nc: Title: Produced by grdmath
cube.nc: Command: Sat Jul 20 16:59:25 2019: ncecat -u level 1.nc 2.nc 3.nc 4.nc cube.nc
grdmath -R0/10/0/10 -I1 X Y MUL = 1.nc
cube.nc: Remark:
cube.nc: Gridline node registration used [Cartesian grid]
cube.nc: Grid file format: nf = GMT netCDF format (32-bit float), CF-1.7
cube.nc: x_min: 0 x_max: 10 x_inc: 1 name: x n_columns: 11
cube.nc: y_min: 0 y_max: 10 y_inc: 1 name: y n_rows: 11
cube.nc: z_min: 1 z_max: 5 z_inc: (variable) name: z n_levels: 4
cube.nc: z_levels: 1, 2, 3, 5
cube.nc: v_min: 0 v_max: 140 name: cube
cube.nc: scale_factor: 1 add_offset: 0
cube.nc: format: classic
cube.nc: Default CPT:

Need to note that we already have a cube file in cache (https://github.com/GenericMappingTools/gmtserver-admin/blob/master/cache/otemp.anal1deg.nc), but the file is too big (~15 MB) which is not suitable for testing purpose.