Resolve "Coherence does not work with MESA whitening"

Closes #39 (closed).

The Issue was raised because no pixel were selected during the clustering phase. The problem was compatibility of w_series with core cwb code. The attribute "wmode" needs to be set to 1, otherwise it would always return 0. The way nRMS were stored in the time-frequency series was wrong, and cWB was not able to read the WSeries object. Now nrms are created via cWB standard procedure and the values are substituted with MESA computed nRMS.