Fix bug in the baseline_spending=True parameterization

[jdebacker]

In PR #880, all TPI output arrays were cast as have a length of T in the time dimension.

This posed an error for the case of baseline_spending=True, which was looking for aggregate transfers (TR) to be of length T+S. This PR fixes that error.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.31%. Comparing base (19c90fc) to head (64d690c).

Files	Patch %	Lines
ogcore/TPI.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
+ Coverage   71.29%   71.31%   +0.01%     
==========================================
  Files          20       20              
  Lines        4954     4953       -1     
==========================================
  Hits         3532     3532              
+ Misses       1422     1421       -1     

Flag	Coverage Δ
unittests	71.31% <0.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
ogcore/TPI.py	35.82% <0.00%> (+0.09%)	⬆️

[rickecon]

@jdebacker. This looks great. Now passed all tests. Merging now.