EVA-1022 Different ploidies should be allowed for a given variant

[jmmut]

following #101 and #102, I took the commits from #103 and finished them:

• Now, in any given row, each sample can have different ploidy.
• As the ploidy affects the cardinality of fields like PL, with this PR each sample can have different cardinality in those fields.
• I kept PL Number=G even in 4.1 and 4.2, because the problem (different samples needed different field cardinality) is fixed by the line above this one.
• Updated tests and documentation accordingly.