Updates to integrate new OPM APIs

[sarthakpati]

Fixes #N.A.

Proposed Changes

• updated API for OPM

Checklist

• I have read the CONTRIBUTING guide
• My PR is based from the current GaNDLF master
• Non-breaking change (would not break existing functionality): please provide as many details as possible for any breaking change
• Function/class source code documentation added/updated
• Code has been blacked for style consistency
• If applicable, version information has been updated in GANDLF/version.py
• If adding a submodule, add to list of exceptions for black styling in pyproject.toml file
• Usage documentation has been updated, if appropriate
• History has been updated, if appropriate
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation

[codecov]

Codecov Report

Merging #261 (a00a07a) into master (5a949da) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   87.23%   87.23%           
=======================================
  Files          80       80           
  Lines        4169     4169           
=======================================
  Hits         3637     3637           
  Misses        532      532           

Flag	Coverage Δ
unittests	87.23% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

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[sarthakpati]

Made a few changes, can you take a look as well, @grenkoca?

[sarthakpati]

Folded into #263 to ensure a single merge.