canvas/germline: Added tests and missing inputs, some cleanup

[Schmytzi]

• Added missing inputs and tests.
• Replaced genomedir input with separate kmer_fasta and genomesize inputs
• Removed automatic selection of ploidy VCF based on sex, now responsibility of the caller
• Removed dependency on external python script
• Replaced file operations on inputs withstageAs
• Added check that at least one SNV VCF is supplied
• Removed automatic filtering of REF calls, now responsibility of the caller
• Replaced hard-coded version with call to Canvas --version

Requires nf-core/test-datasets#2188 to be merged first.

PR checklist

• This comment contains a description of changes (with reason).
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