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 catch. This is not used in anywhere yet. "is_splice_site" is used in filter code. I just meant to encapsulate the @splice_sites for the future possible external access.