Use Systems and remove dependency on SemialgebraicSets

[blegat]

Closes #2 cc @mforets @schillic

[blegat]

@mforets Glad to hear that DiscreteIdentitySystem is welcome in Systems.jl :)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@3521ffc). Click here to learn what that means. The diff coverage is 18.18%.

@@            Coverage Diff            @@
##             master       #5   +/-   ##
=========================================
  Coverage          ?   12.62%           
=========================================
  Files             ?        6           
  Lines             ?      103           
  Branches          ?        0           
=========================================
  Hits              ?       13           
  Misses            ?       90           
  Partials          ?        0

Impacted Files	Coverage Δ
src/switchedsystems.jl	0% <0%> (ø)
src/HybridSystems.jl	40% <40%> (ø)

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[schillic]

Meta comment: I learned two things from this PR. 👍

• Write functions for tests that you then call with different arguments.
• Github recognizes the "closes" keyword.

[blegat]

The first one is something we use extensively in MOIT :-P