Allow contexts restricted to a smaller domain

[fredrik-johansson]

Currently a ca_t allows representing any complex number together with infinities. It should be possible to initialize context objects that restrict computation to a smaller domain:

• Algebraic numbers
• Real algebraic numbers
• Real algebraic numbers + infinities
• Real numbers
• Real numbers + infinities
• Complex numbers
• ...

Anything outside the supported set should give Undefined. For example, whereas sqrt(-1) -> i by default, one should have sqrt(-1) -> Undefined when working strictly with real numbers.