fix TypeError in format_chunksize, y-axis scale of knee-plot

[ChangqingW]

Fix TypeError when chunksize is specified as int

File "/LR-splitpipe/LR-splitpipe/demultiplex.py", line 332, in format_chunksize
    if '**' in c:
TypeError: argument of type 'int' is not iterable

[ChangqingW]

The y-axis title of the knee plot says log scale but log scale was not applied:

Added a commit to make it log scale: