Parametrised intervals

Interval arithmetic rewritten to use parametrised intervals.
Tests pass.
Notes on ⊆ and ⊊. Check the following:
```julia
julia> a = @interval 0 0.1
[0e+00, 1.0000000000000001e-01] with 53 bits of precision
julia> 0.1 ⊆ a   # OK with the answer, but perhaps the left side should be an interval
true
julia> @which 0.1 ⊆ a
issubset(l,r) at set.jl:96
julia> 0.1 ⊂ a   # Using these symbols, this should be included (\subset<TAB>)
ERROR: ⊂ not defined
julia> 0.1 ⊊ a 
ERROR: `⊊` has no method matching ⊊(::Float64, ::Interval{BigFloat})
# The latter should behave as close as possible as to ⊆
```
I prefer to use ⊂ rather than ⊊, though I understand the point of including it. I think we can simply add ⊂ to do just the same as ⊊. What do you think? Moreover, we should have similar behavior of ⊆ and ⊊, which ideally would need an interval on the left hand-side of the operator.