Improved type hierarchy for truncation

[THargreaves]

The current type hierarchy is not sufficiently expressive.

The main issues arise when composing Lévy process, e.g. through subordination. We have that,

const TruncatedSubordinator{P<:Subordinator,T<:Real} = TruncatedLevyProcess{P,T}

but this then means that a TruncatedSubordinator does not know it is a Subordinator.

A better approach could be to keep track of the domain of the Lévy measure for each process, truncating this when the jump sizes are truncated. This can no longer be dispatched on, but they values could be validated.