Loosening the restriction on index type

[asinghvi17]

We should also open up the index type to Real values - this would allow people to load arbitrary results which have some time-like variable.

More generally, I don't think there is a reason that we shouldn't accept any type for which Base.lt(x::T, y::T) is defined (meaning it can be sorted). Julia is all about generality, and this would allow people to use e.g. Unitful.jl numbers as indices. Plus, all our algorithms should be generic enough not to be worried about the index type anyway.

[chiraganand]

Separate similar discussions going on in #167 and #151.