New, improved version

I created a new version of this module that:

• leverages Core's numeric field types and formatters, thereby creating database tables of the correct numeric type (instead of string type) as well as adding type-specific features such as prefix/suffix, scale, a choice of decimal and thousands markers etc.
• brings the module back on par with D7's Math field → , particularly the support of variables and functions such as f(x) = x + 3; f(2)

Following some more testing and maybe even some test coverage, I believe this approach should either become a new 8.x-2.x-dev branch, or should be re-merged with the original Math field → project which currently lacks a 8.x version.