v0.3.0 Better joint calling for larger cohorts

Major:

• When doing joint calling, merge subcommand only reports bounds with a minimum number of supporting reads in at least one sample (default: 6)
• Introduces index subcommand to create an str index of the reference genome. This was previously done per sample, but is more efficient to do once in advance for larger cohorts

Minor:

• Memory required for merge reduced by more than half
• Fix int overflow bug
• Docs updates