Allow non-literal length scale for quasiperiodicity

For the process qp(P, ell, n, rho, sigma), ell currently must be a numeric literal or a variable whose definition is a numeric literal. Extend things so that ell need not be known when you compile the model, but can be an argument passed in. That is, the compiled Stan model should allow ell to be passed in via the data section or computed in the transformed data section.

No file or test is named. Trace the compiler handling of qp(P, ell, n, rho, sigma), starting at the check that requires ell to be a numeric literal or literal-valued variable. Done means a compiled Stan model accepts ell from the data section or computes it in transformed data without requiring its compile-time value.

Written by the indexing model from the issue text.