Separate TimeSeries union type

[rusandris]

There should be obvious type notations for methods that work on real-valued datasets or symbolic datasets.
Right now, there's a union type TimeSeries{T,D} = Union{StateSpaceSet{D,T},AbstractArray{T,D}} where {T,D} for convenience that stands for both (and not even exported).

• timeseries_to_grid methods should stay just Vector{Float64},Matrix{Float64},StateSpaceSet{D, Float64}, possibly with a type parameter T <: AbstractFloat to accomodate Float32 measured time series data.
• calculate_transition_matrix should stay generic enough to be able to handle a wide range of possible symbol types.