Add getter function for assignments

[mforets]

Proposal: add "assignment" function as the getter function for reset maps:

assignment(hs::HybridSystem, t) = hs.resetmaps[symbol(hs, t)]

Note. If we add "ConstrainedMap" as proposed MathematicalSystems#60, then it is implemented as

assignment(hs::HybridSystem, t) = hs.resetmaps[symbol(hs, t)].map

Either way, i think it is useful to add this function because it is less implementation-dependent.

[schillic]

The first one is confusing because it does not give you the assignment.
The second one is fine as long as you dispatch on the type of resetmaps such that you know that there is a map field.