Copy pvlib's SPA (NREL's) implementation

[echedey-ls]

Closes #94

For extra control over the source code (and the docstring!), and easing a pure-python reference for translating into other languages. Numba has been stripped off, plus adding some other code stylisation and opinionated best practices.

Feel free to make any and all objections.

[AdamRJensen]

There are a lot of functions in this PR. Should most of them be made private functions?

@kandersolar How should we give credit / copyright to pvlib/you?

[kandersolar]

If this code was modified from pvlib's code, we will need to also include pvlib's license, per this clause:

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this
list of conditions and the following disclaimer.

[echedey-ls]

I midly changed my mind about having stand-alone implementations. We aren't going to rewrite others too and, except for that performance PR at pvlib, both algorithm and interface should be pretty stable.

I guess I can instead wrap pvlib's call into a locally-documented public function and stop there. Pretty much like it has been made to other external-dep algorithms. While not cleaner (the code, I mean, with the numba decorators), it shouldn't be difficult to translate to other languages by navigating to the source. So, unless you object, I will change the scope of this PR, keep the fixed typo, link to pvlib's source, and unpin pvlib version from the docs optional dependency set.

[AdamRJensen]

I midly changed my mind about having stand-alone implementations. We aren't going to rewrite others too and, except for that performance PR at pvlib, both algorithm and interface should be pretty stable.

I guess I can instead wrap pvlib's call into a locally-documented public function and stop there. Pretty much like it has been made to other external-dep algorithms. While not cleaner (the code, I mean, with the numba decorators), it shouldn't be difficult to translate to other languages by navigating to the source. So, unless you object, I will change the scope of this PR, keep the fixed typo, link to pvlib's source, and unpin pvlib version from the docs optional dependency set.

Sounds great to me!