Improve pvlib.spa speed (#1748)

* initial optimization

* combine longitude and obliquity nutations

* minor optimization to heliocentric latitude

* reuse memory in several places

* remove unused function

* minor optimizations (reuse calls to radians())

* Revert "reuse memory in several places"

This reverts commit 75e6265.

* numba fix

* fix tests

* whatsnew

* whatsnew

* bugfix

* don't combine coefficient arrays into a common table

* factor out common calculation

* ease back claimed improvement in whatsnew

* stickler

* work around strange numba issue (/bug?)

* stickler and fix test oversight

* updates from review

* less is more

Co-Authored-By: Will Holmgren <[email protected]>

---------

Co-authored-by: Will Holmgren <[email protected]>

Author: kandersolar

docs/sphinx/source/whatsnew/v0.9.6.rst

@@ -37,6 +37,7 @@ Enhancements
   the option of controlling what NSRDB endpoint is used. (:pull:`1736`)
 * :py:func:`pvlib.iotools.get_psm3` now uses the new NSRDB 3.2.2 endpoint for
   hourly and half-hourly single-year datasets. (:issue:`1591`, :pull:`1736`)
+* The default solar position algorithm (NREL SPA) is now 50-100% faster. (:pull:`1748`)
 
 Bug fixes
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