Fixed issues with MR tests for SSR dataset

- implemented and tested functionality of distance measures to compute
overall score for entire dataset if no solution given (null)

- renamed AccessionSSRMarkerMatrixListImplTest to
AccessionSSRMarkerMatrixListImplWrapperForTests

- updated tests in TestModifiedRogersDistanceSSR: no longer rely on
absolute indices of accessions -- request indices based on accession
names