fix(pdesystem): parameters are vector of Num instead of vector of pairs when analytic is also passed

[sathvikbhagavan]

Parameters are vector of Nums. If we pass a vector of Nums when we pass analytic as well, it fails.

MWE:

using ModelingTookit, DomainSets

@variables x t u(..)
@parameters D

Dxx = Differential(x)
Dt = Differential(t)
eqs = [Dt(u(t, x)) ~ D * Dxx(u(t, x))]
bcs = [u(0, x) ~ sin(2pi * x), u(t, 0) ~ 0.0, u(t, 1) ~ 0.0]

domains = [t ∈ Interval(0.0, 1.0), x ∈ Interval(0.0, 1.0)]

analytic = [u(t, x) ~ exp(-4pi^2 * D * t) * sin(2pi * x)]
@named heat_1d1 = PDESystem(eqs, bcs, domains, [t, x], [u(t, x)], [D], analytic = analytic) # fails
@named heat_1d1 = PDESystem(eqs, bcs, domains, [t, x], [u(t, x)], [D => 1.0], analytic = analytic) # passes
@named heat_1d1 = PDESystem(eqs, bcs, domains, [t, x], [u(t, x)], [D]) # passes

[sathvikbhagavan]

@ChrisRackauckas, can this be merged?