Add IsNumber trait, extending std::is_arithmetic

Since it's very difficult to make progress on !52 (merged), I'll start to break it up into smaller, less controversial changes. This is the first piece that I'd also like to use in other places. An IsNumber trait allows us to determine when we've hit the bottom in a recursion without a blocklevel and things like FieldVector<ctype, 1>.

IsNumber is currently true for: floating point types, gmp fields, and such types wrapped in std::complex.