Add resetmap getter

[mforets]

This is a rework of #22.

Note that i added the type annotation R<:AbstractMap in HybridSystem. The motivation is that if we are going to let assignment be the function, then it cannot be Union{AbstractDiscreteSystem, AbstractMap} as implied in the previous doc, because the discrete systems do not have an "apply" method.

If you're not convinced i can revert it, letting assignment work only when:

assignment(hs::HybridSystem{A, S, R, W}, t) where {A, S<:AbstractSystem, R<:AbstractMap, W} = ...

I also added the type annotation S<:AbstractSystem in HybridSystem. Becaue we have methods such as inputset, statedim and so on that make sense if the vector of modes are systems.

[mforets]

Note that i added the type annotation R<:AbstractMap in HybridSystem.
I also added the type annotation S<:AbstractSystem in HybridSystem.

I take back these. Since they are not strictly needed for this PR, for now let's just make assignment work for maps.

[schillic]

See my comment here.

[codecov]

Codecov Report

Merging #25 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   81.36%   81.36%           
=======================================
  Files           6        6           
  Lines         161      161           
=======================================
  Hits          131      131           
  Misses         30       30

Impacted Files	Coverage Δ
src/HybridSystems.jl	73.33% <100%> (ø)	⬆️

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[mforets]

Ready. Waiting for review.