Add modulator

[kklurz]

• This PR adds an MLP modulator as used in Toliaslab. The version from Toliaslab is changed to a) have a similar structure as the shifter module from neuralpredictors and b) to allow the modulator to be used by models with more than one output (like Zero Inflated Distributions as opposed to Poisson)
• The PR also changes the NeuroNormalizer to have the option to center the behavior variables around zero, i.e. to subtract the mean before variance normalizing. While this does not make sense from a physical perspective (pupil dilation for instance can not be negative and should not be centered around zero), it is necessary for the modulator network: Passing in "0" should result in no modulation.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.09 ⚠️

Comparison is base (4a32cbf) 8.44% compared to head (35dba16) 8.35%.

❗ Current head 35dba16 differs from pull request most recent head a64fb3d. Consider uploading reports for the commit a64fb3d to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #190      +/-   ##
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- Coverage   8.44%   8.35%   -0.09%     
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  Files         50      51       +1     
  Lines       5009    5060      +51     
  Branches     833     840       +7     
========================================
  Hits         423     423              
- Misses      4557    4608      +51     
  Partials      29      29              

Impacted Files	Coverage Δ
neuralpredictors/data/transforms.py	0.00% <0.00%> (ø)
neuralpredictors/layers/modulators/mlp.py	0.00% <0.00%> (ø)
neuralpredictors/layers/shifters/mlp.py	0.00% <0.00%> (ø)

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[mohammadbashiri]

Very nice. Thanks!