Basis function latent process models

[SamuelBrand1]

Noting this issue which is a feature of EpiNow2 epiforecasts/EpiNow2#757 . We should aim to have this functionality in EpiAware.

##Update

I've widened the scope of this issue because the same type of Latent process covers Bayesian splines as well as Hilbert space approximated GPs.

Thoughts

• We'd want a constructor that only one time calculates the matrix projections of basis functions onto our integration domain (probably usually daily).
• Helper functions for common examples of this: e.g. sin cos basis projections, also Splines2.jl can do spline basis function projections.
• Priors, these are known for the hilbert space approx