Number: Move format integer fraction

Move format integer fraction digits into its own file and unit test it.

Ref gh-200, gh-214
Number: Significant digits

Ref gh-200
Closes gh-214
(fix) Number: Significant digits

Fixes error message:
-               throw new Error( "Ops! Integers out of (x^20, x^-7) not supporte
+               throw new Error( "Ops! Integers out of (1e21, 1e-7) not supporte