Allow input and output scalar type to differ for functions

[a7g4]

vague/include/vague/differentiable_function.hpp

Line 35 in 5e1e871

// TODO: Relax this requirement - functions should be able to go from one scalar type to another

vague/include/vague/arbitrary_function.hpp

Line 31 in 5e1e871

// TODO: Relax this requirement - functions should be able to go from one scalar type to another