Generate data histograms to help determine the scale type

[arteymix]

It's often difficult to tell the scale type of data, especially single-cell data. In my case, I have to load the MEX files that Gemma generated back to disk and inspect them manually.

Gemma could include a data histogram plot in its QC plots. It should be available when editing QTs.

• generate histogram per sample and for all samples combined
•  recommend a QT type as per our detection framework