Refactor create_coverage.* plots

[stefaneng]

• Use BPG::write.plot to write plot if filename is not NULL
  • This needs to happen for the plots to be successfully added together when filename is not null.
• Remove duplicated code in create_coverageplot.HistogramFit for base coverage plot with a call to create_coverageplot.Histogram (without filename passed)
• Pull fitted distribution plot into own function to use in create_histogram code