removing requirement for bam_readcount_version param - default will be set in

GMT::Sam::Readcounts and propogate up, unless otherwise specified
Store the common params in an array or hash and then add the bam_readcount_version key/value if it's specified.

```
my @params = (
    bam_file => $bam,
    ...
);
if ($self->bam_readcount_version) {
    push @params, bam_readcount_version => $self-> bam_readcount_version;
}
$cmd = Genome::Model::Tools::Analysis::Coverage::BamReadcount->create(@params);
```