Handle properly Number abstraction and concrete types Real and Integer

[Tomaqa]

Currently, everything is actually a real type so computation with integers is inefficient.
Moreover, sometimes the types may be used interchangeably although an explicit conversion would be more appropriate.

Consider using std::variant, maybe even with CRTP for defining a common interface of reals and integers, but it should not be necessary.
https://gist.github.com/olibre/3d0774df0f7a16e2da10fae2b2f26c4f