Refactor VepInterpreter.pm and add SpliceNonsynonymousFilter.pm

Refer to AT-628. For ACMG report, a new vep filter is needed to remove all variants that hit neither splice sites nor non-synonymous coding sites.
Good suggestion. I once thought of that, but my hesitation is that those two categories are all they need. Anyway, I will refactor this to be more generic.