feat: adding Identity PSF modality

[federico-carrara]

Added Identity as a new _PSFModality.

• What: this new modality skips PSF convolution. Hence, the optical image corresponds to the so-called "emission flux".
• How: by overriding the render() method of _PSFModality. Now it simply multiplies the GT array by the emission spectra for the different channels and FPs.
• Why: we need this modality in the case the ground truth (FP distrib) is obtained from a light microscopy image (i.e., already convolved by the PSF). For instance, we can have this situation when we want to simulate a spectral image staring from a single frame.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.83%. Comparing base (4869b82) to head (3a26a35).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/microsim/schema/modality/_simple.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   84.87%   84.83%   -0.05%     
==========================================
  Files          47       47              
  Lines        3121     3125       +4     
==========================================
+ Hits         2649     2651       +2     
- Misses        472      474       +2     

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[tlambert03]

thanks!