dist changes!

This contains a bugfix for a very rare (but major) bug that occurs when successive chromosomes have the same length. The data from the first chrom was not cleared and then polluted the counts for the subsequent chrom. Thanks to Kate B. for reporting and providing a simple test-case.

It also changes the dist output file name(s). Before only a single dist file was created. Now, there will always be a $prefix.global.dist.txt and if --by is specified, there will also be a $prefix.region.dist.txt. Thanks to Alistair W for suggesting.

See below for more details.

0.2.2

• fix overflow with huge intervals to --by
• NOTE change to output file name of *.dist.txt. A file named $prefix.mosdepth.global.dist.txt
  will always be created and $prefix.mosdepth.region.dist.txt will be created if --by is specified.
  Previously, there was only a single file named $prefix.mosdepth.dist.txt which no longer exists.
  This allows users to, for example, use --by to see coverage of gene regions for WGS, and to see the
  global WGS coverage and the coverage in their genes of interest.
• fix bug that would manifest with consecutive chromosomes of the same length. chromosomes other than
  the first of a given length would have incorrect values.