Make pvlib an optional dependency

[AdamRJensen]

Currently, solposx has three dependencies:

• numpy: 12.5 MB
• pandas: 10.9 MB
• pvlib: 19.4 MB

All functionality depends on numpy. The timestamp handling requires pandas. pvlib is the package with the largest download footprint, and is not required for most functions.

I would like to make pvlib an optional package. My motivation is to make loading solposx faster, which is important for lightweight applications, such as interactive examples using marimo.

I imagine the process would be something like:

• Step 1: add sind, cosd, tand, asind, and acosd to tools.py instead of importing from pvlib
• Step 2: consider copying pvlib.spa.calculate_deltat to solposx (used by noaa and usno)
• Step 3: add optional dependency warning to solposx.solarposition.pvlib