cnvkit2theta.py, theta2cnvkit.py: New scripts for interop with THetA

See: http://compbio.cs.brown.edu/projects/theta/

Respectively, these scripts (1) generate the THetA input file, bypassing the
exome import thing, and (2) convert the THetA results file to a .cns-like file
where the log2 ratio values are instead the absolute copy number.