Spectral mismatch calculation code (#1524)

* Add mismatch module in pvlib.spectrum

* Add utility function to produce a sample sr to work or play with.

* Add reference spectrum file and reader function.

* Add the mismatch calculation.

* Add tests and expand doc strings.

* Update whatnew and placate Stickler.

* Change am15 file from xls to csv; doc string expansion and misc small changes.

* Adjust __init__.py and imports; sphynx references; whatsnew.

* Undo whatsnew v0.9.2

* Resolve multiple issues from ongoing review.

* Make many improvements based on reviews.

* Fix indentation and __init__.py

* Polish docstrings some more.

* Add explanation of truncation strategy.

* Final touches.

* Expand explanation for truncation

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>

Author: adriesse

docs/sphinx/source/reference/effects_on_pv_system_output.rst

@@ -49,3 +49,6 @@ Spectrum
    :toctree: generated/
 
    spectrum.spectrl2
+   spectrum.get_example_spectral_response
+   spectrum.get_am15g
+   spectrum.calc_spectral_mismatch_field

docs/sphinx/source/whatsnew/v0.9.3.rst

@@ -12,6 +12,11 @@ Enhancements
 * Low resolution altitude lookup map
   :py:func:`~pvlib.location.lookup_altitude`
   (:issue:`1516`, :pull:`1518`)
+* New module to calculate spectral mismatch from field spectral measurements
+  :py:func:`~pvlib.spectrum.mismatch.get_example_spectral_response`
+  :py:func:`~pvlib.spectrum.mismatch.get_am15g`
+  :py:func:`~pvlib.spectrum.mismatch.calc_spectral_mismatch_field`
+  (:issue:`1523`, :pull:`1524`)
 * Added Townsend-Powers monthly snow loss model:
   :py:func:`pvlib.snow.loss_townsend`
   (:issue:`1246`, :pull:`1251`, :pull:`1468`)
@@ -40,5 +45,10 @@ Contributors
 ~~~~~~~~~~~~
 * João Guilherme (:ghuser:`joaoguilhermeS`)
 * Nicolas Martinez (:ghuser:`nicomt`)
+* Anton Driesse (:ghuser:`adriesse`)
+* Cliff Hansen (:ghuser:`cwhanse`)
+* Kevin Anderson (:ghuser:`kanderso-nrel`)
+* Mark Campanelli (:ghuser:`markcampanelli`)
+* Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Abhishek Parikh (:ghuser:`abhisheksparikh`)
 * Taos Transue (:ghuser:`reepoi`)