Ensure synthesis metrics have an option to take voided image

[sarthakpati]

Fixes #957

Proposed Changes

• the synthesis metrics for BraTS rely on something called a "void image", which was not accounted for before.
• it is now possible to either use a mask or a "void image" for normalization (sarthakpati@1c4afd7)
• updated ncc metrics to get results from a masked region
• updated spellings and typos

Checklist

• CONTRIBUTING guide has been followed.
• PR is based on the current GaNDLF master .
• Non-breaking change (does not break existing functionality): provide as many details as possible for any breaking change.
• Function/class source code documentation added/updated (ensure typing is used to provide type hints, including and not limited to using Optional if a variable has a pre-defined value).
• Code has been blacked for style consistency and linting.
• If applicable, version information has been updated in GANDLF/version.py.
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file.
• Usage documentation has been updated, if appropriate.
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation.
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3].
• The logging library is being used and no print statements are left.

[github-actions]

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[szmazurek]

Just one comment about possibly separating functions.