fix bug in importer and revisit ref genome in validator

[SRodenburg]

The validator and importer Java backend code are rather confusing on what syntax to use to indicate an alternative genome build (e.g. GRCh38/hg38 or GRCm38/mm10 for mouse). There is a mixed and ambiguous usage of the terms "genome build" and "reference genome" throughout the code.

• Validator now checks that meta_study.txt in (optional) field reference_genome contains one of hg19, hg38, mm10 and else uses the values specified in portal.properties.
• Validator now checks for MAF files that the column NCBI_Build contains a value that is consistent with the specified reference_genome value, thus GRCh37 for hg19 studies etc.
• some code cleaning to make this logic better comprehensible.

Already some time ago I looked into this but we never sorted this out: cBioPortal/cbioportal-frontend#3073

There is also an accommodating PR from @pvannierop in frontend. (cBioPortal/cbioportal-frontend#4084)

This partially fixes this issue as well: cBioPortal/icebox#115

[sonarcloud]

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[SRodenburg]

@inodb happy new year! Can we merge this one?

[inodb]

@SRodenburg happy new year! Can this be merged independently of the other PRs mentioned in the description (e.g. the frontend)