Support Interval Taylor models

[lgeretti]

We shall enable some factory, supplied as configuration for an integrator, that will allow us to produce singleton or interval coefficients for the functions involved.

[pietercollins]

"Interval" Taylor model has two semantics. Using FloatBounds as coefficients gives an ordinary ValidatedFunction, which should slot directly in to existing solvers. (But the TaylorModel code still needs to be written.) Using UpperInterval as coefficients gives a multivalued function.