Allow multiple filters

[JeremyWesthead]

Should fix 2 edge cases.

1. 3 years ago in my initial implementation I made the assumption of only a single filter per VCF row. This isn't true and the assert statement to catch this was checking for >= 0 rather than > 1 so this was never caught. Should now parse VCF filters as a list (as it always should have)
2. Null calls were being allowed through in cases of >1 filter fail if the first filter was MIN_FRS. This was due to only parsing (and so checking) the first filter. Should be fixed as part of this

Written relatively late (as it suddenly occurred that this was happening and needs fixing before release) so please double check my logic!

[philipwfowler]

Checked logic and ran tests locally, including modifying TEST-DNA-2.vcf so FILTER was a list MIN_GCF;MIN_FRS and test failed as expected. Code does assume that (i) a row that passes only contains PASS in the FILTER and (ii) a null row that only contains MIN_FRS would otherwise have passed. Both are reasonable assumptions.