Update function genfromtxt: add keyword `nrows`

Add keyword `nrows` to specify the number of rows to read.
1. We should not use keywords `nrows` and `skip_footer` in the same time. 
2. If there's not enough lines (less than nrows) to read, an exception would raise.
I don't think that `np.inf` is guaranteed to be a singleton, so comparisons using `is` are risky at best. My vote for using `None` directly also applies here.