Add site-level PV input subnetwork

[dfulu]

Pull Request

Add support to the multimodal model to optionally include an encoder for system-level PV inputs

• Add a couple of simple PV site-level encoder architectures
• Add PV site encoder to multimodal model
• Slight refactor of multimodal model and configs
• Update tests

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

Merging #88 (293328d) into main (80222f5) will decrease coverage by 13.63%. The diff coverage is 65.00%.

:exclamation: Current head 293328d differs from pull request most recent head 4cc54b8. Consider uploading reports for the commit 4cc54b8 to get more accurate results

@@             Coverage Diff             @@
##             main      #88       +/-   ##
===========================================
- Coverage   64.18%   50.55%   -13.63%     
===========================================
  Files          21       23        +2     
  Lines        1477     1614      +137     
===========================================
- Hits          948      816      -132     
- Misses        529      798      +269     

Files	Coverage Δ
pvnet/models/multimodal/encoders/basic_blocks.py	95.12% <100.00%> (+0.25%)	:arrow_up:
pvnet/models/base_model.py	35.48% <0.00%> (ø)
pvnet/models/multimodal/multimodal.py	95.60% <96.00%> (-1.31%)	:arrow_down:
...et/models/multimodal/site_encoders/basic_blocks.py	90.90% <90.90%> (ø)
pvnet/utils.py	22.00% <50.00%> (ø)
pvnet/models/multimodal/site_encoders/encoders.py	95.83% <95.83%> (ø)
pvnet/optimizers.py	25.00% <16.41%> (-28.34%)	:arrow_down:

... and 4 files with indirect coverage changes

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