#961 - Make low/high of Interval faster

[mforets]

Closes #961.

Let

julia> using LazySets, BenchmarkTools

julia> x = Interval(0.9, 1.1)
Interval{Float64,IntervalArithmetic.Interval{Float64}}([0.9, 1.10001])

Master:

julia> @btime low($x)
  97.383 ns (3 allocations: 288 bytes)

1-element Array{Float64,1}:
 0.8999999999999999

This branch:

julia> @btime low($x)
 28.365 ns (1 allocation: 96 bytes)
1-element Array{Float64,1}:
0.9

[schillic]

Great, you also found a floating-point issue here 👍