Write methylation coverage analysis module

[ewels]

• Move the current coverage code out of bismark_methXtract module and in to own module.
• Move the genome wide coverage report stuff from bismark_methXtract and just run coverage2cytosine instead.
• Use new code written into ngi_visualizations for coverage analysis.

[ewels]

Split out into it's own module in 1a7cb50

[ewels]

New ngi_visualizations script in progress.

[ewels]

Removed half-finished bismark_coverage module from master branch in 82afc08 and moved into the bismark_coverage branch for development.