Improve/is number formula

Basically, checking for a number formula is more reliable than looking
for a string formula.

The main requisite for a number formula is:

- Is a NSNumber
- Is a NSString that contains 0123456789
Fixed https://github.com/hyperoslo/NSString-HYPFormula/commit/1d17cfea68d0103e2bd8321c5570429ab8b88cc5