Remove unneeded .isNumber calls

Vows .epislon now checks if the argument is a number, no longer
necessary.

PR-URL: https://github.com/jstat/jstat/pull/119
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
fix the centralF Distribution x = 0

Add tests for the central f distribution's pdf where x = 0.0. There are
two special cases that are not handled by the general form of the
central F distribution's pdf (df1 = 1 and df2 = 1; df1 = 2 and df2 = 1);
handle these with specific checks.

Fixes: https://github.com/jstat/jstat/issues/97
PR-URL: https://github.com/jstat/jstat/pull/119
Reviewed-by: Trevor Norris <trev.norris@gmail.com>