Add command-line option for importing only a given number of rows

Some data file are very big, and contains a very high number of rows.

Users should be able to specify a maximum number of rows to be read, excluding skipped rows at the beginning of the file. The option is already offered by numpy.genfromtxt's max_rows, but not be numpy.loadtxt (cf. https://docs.scipy.org/doc/numpy/reference/generated/numpy.genfromtxt.html#numpy.genfromtxt).

It could be added as an command-line option --max-rows.