Add INT algorithm

[mforets]

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[mforets]

xref: JuliaReach/Reachability.jl#671

This is now the INT algorithm in RA. The motivation is to have an efficient method for constructing the linear part of flowpipes in nonlinear problems. I tested prob = @ivp(x' = -x, x(0) ∈ Interval(0.45, 0.55)), the results are

alg	opts	btime
(RA) GLGM06(δ=0.01), old	T=4.0,	223.894 μs (2138 allocations: 125.55 KiB)
(RA) INT(δ=0.01), new	T=4.0,	6.275 μs (18 allocations: 13.83 KiB)
(RA) GLGM06(δ=0.01), new + improved discretization	T=4.0	199.254 μs (2023 allocations: 117.06 KiB)
(RA) GLGM06(δ=0.01), new + w/inference fixes	T=4.0	98.471 μs (1617 allocations: 104.41 KiB)`
(R) BFFPSV18(δ=0.01)	T=4.0	605.488 μs (13739 allocations: 660.63 KiB)

where RA : ReachabilityAnalysis / R: Reachability. For 1d systems, INT is the new default.