Add functionality to use linear interpolation to fill gaps in input data

[dfulu]

Pull Request

• This adds functionality to SelectTimeSliceIterDataPipe to allow linear interpolation infilling of missing values up to a maximum number of consecutive time steps.
• Tests added for this functionality
• Added to pvnet production datapipe

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 #230 (c7d8da3) into main (dbd6e8d) will increase coverage by 0.21%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
+ Coverage   79.73%   79.94%   +0.21%     
==========================================
  Files         126      126              
  Lines        5507     5551      +44     
==========================================
+ Hits         4391     4438      +47     
+ Misses       1116     1113       -3     

Files	Coverage Δ
ocf_datapipes/select/select_time_slice.py	100.00% <100.00%> (ø)
ocf_datapipes/training/pvnet.py	82.02% <ø> (ø)

... and 1 file with indirect coverage changes

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