Error for read length related plots (numpy.linalg.LinAlgError: Singular matrix)

[yfukasawa]

Self posting.

Issue happened in qscore and length_vs_coverage plotting while analyzing datasets having very small variance in read length.
The cause of the error was that the number of bins for read length can be very small in such a case (< 5).
Checking the number of bins before plotting must be done.

Y.