FIX: Address memory issues and corruption in fMRIPlot

[oesteban]

At the same time, this PR ensures that all calls to plt.savefig() are followed by closing the figure.

Resolves: #129.
Closes: #130.

[codecov]

Codecov Report

Attention: Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.

Project coverage is 59.21%. Comparing base (e0a30e6) to head (065fd2a).
Report is 51 commits behind head on main.

Files with missing lines	Patch %	Lines
nireports/reportlets/modality/func.py	45.45%	4 Missing and 2 partials ⚠️
nireports/interfaces/dmri.py	0.00%	2 Missing ⚠️
nireports/interfaces/fmri.py	0.00%	1 Missing ⚠️
nireports/reportlets/nuisance.py	93.75%	1 Missing ⚠️
nireports/reportlets/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   59.20%   59.21%   +0.01%     
==========================================
  Files          20       20              
  Lines        2238     2256      +18     
  Branches      398      402       +4     
==========================================
+ Hits         1325     1336      +11     
- Misses        814      820       +6     
- Partials       99      100       +1     

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