Bug fix => to = and refactoring of parameter construction for BamReadcount

There is a bad typo from PR #94  that code review missed. This bug causes snv bam readcount uses default 0.6 version and indel bam readcount uses 0.5 version. This change fixes the bug and refactors the parameter construction for snv/indel. This module itself might need more refactoring and need a real unit test, but it is out of the scope of this pull request. This change need to be hotfix in genome-stable ASAP.