Bugfix for filter which was too strict in exceptional cases.

Concerns filter for selecting which markers to draw if there are more than user allows to be drawn.
In the exceptional case that the cutoff_sd value was not unique in sd_rows_mRNA, the original code would include fewer markers than specified (data with identical sd values would even result in zero markers making the cut off). New code will always include the allowed number of markers to be drawn, ordered by highest sd value first and order of the input data second.