Fixes #52 (infinite loop for fractional numbers)

[markus-willems]

I added a check for fractional numbers to the permutation function:

if (nelem % 1 !== 0) throw new RangeError;

It (P function line 26) would otherwise enter an infinite loop for e.g. nelem = 1.5.

I added tests for this case, too.

@dankogai could you please review this PR? Thanks! ☺️

[markus-willems]

I have just noticed it's the same for combination. I will add a check and test for that, too.

Going to close this PR for now.