ENH: Add session-label option

[mnoergaard]

This PR addresses issue #167 by implementing the possibility to carry out session-wise processing. This may be required in cases such as baseline-blocked designs, where the signal may rapidly change following the blocking. This may influence the performance of motion correction and co-registration, which may need specialized attention.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.66%. Comparing base (047c5fa) to head (db25bbd).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
petprep/cli/parser.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
+ Coverage   79.56%   79.66%   +0.10%     
==========================================
  Files          82       82              
  Lines        6106     6138      +32     
  Branches      614      618       +4     
==========================================
+ Hits         4858     4890      +32     
  Misses       1092     1092              
  Partials      156      156              

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