CnvReferenceCohort: Mean correlation

[leonschuetz]

Mean correlation calculation fails if one coverage file returns nan for correlation.

leads to pipeline crash since mean correlation will not writen to CNV file and TsvToQC fails:
Exception: Input line with more/less than two parts: 'QC:2000114'

Example: 23006a003-PCR5_01 (sample has no reads, so correlation is nan (removed from coverage folder as quick fix))

[marc-sturm]

should be fixed with e3e4d6d