Add colorbar to ALFF and ReHo plots

[tsalo]

Closes #627.

Changes proposed in this pull request

• Add a colorbar to the volumetric ALFF and ReHo plots.
• Close figures after saving them to prevent bleedover.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.51%. Comparing base (ec51642) to head (e824db7).
Report is 149 commits behind head on main.

Files with missing lines	Patch %	Lines
xcp_d/utils/plotting.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1023   +/-   ##
=======================================
  Coverage   81.51%   81.51%           
=======================================
  Files          47       47           
  Lines        5604     5611    +7     
  Branches      743      743           
=======================================
+ Hits         4568     4574    +6     
- Misses        850      851    +1     
  Partials      186      186           

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