Add support for transition_duration for transitions

[blegat]

Consider the switched systems characterized by the graphs G1 and H3 of the examples https://github.com/blegat/SwitchOnSafety.jl/blob/master/examples/AJPR14e54.jl
The time duration of some of the transitions is 2 so this line:

SwitchOnSafety.jl/src/scaled.jl

Line 42 in 1a62c7e

scale(HybridSystems.resetmap(hs.system, t), hs.γ)

is not correct.
To fix this, we will assume that if there is no entry :transition_duration in the ext Dict:
https://github.com/blegat/HybridSystems.jl/blob/46d60438a2ae196a740274fb416f2093fe104547/src/HybridSystems.jl#L63
then the transition_duration is 1 second for each transition.
Otherwise, ext[:transition_duration] should contain the time duration of each transition.
So for instance, to construct the graph G1 of the example, one would do

transition_duration = HybridSystems.transition_property(G1, Float64)
transition_duration[HybridSystems.OneStateTransition(1)] = 1.0
transition_duration[HybridSystems.OneStateTransition(2)] = 1.0
transition_duration[HybridSystems.OneStateTransition(3)] = 2.0
G1.ext[:transition_duration] = transition_duration

Then the line

SwitchOnSafety.jl/src/scaled.jl

Line 42 in 1a62c7e

scale(HybridSystems.resetmap(hs.system, t), hs.γ)

would be replaced by

transition_duration = get(hs.system.ext, :transition_duration, nothing)
if transition_duration === nothing
    scaling = hs.γ
else
    scaling = hs.γ^transition_duration[t]
end
scale(HybridSystems.resetmap(hs.system, t), scaling)

Once this is implemented, it should be tested by adding the graph G1 and H3 to the example.