Make MCMCMultiProposal defaults constructible

[BJMCox]

MCMCMultiProposal only accepts proposal vectors, but its default proposal value is a tuple.

using BAT

MCMCMultiProposal((MetropolisHastings(),), [1])

On d33ddf2b:

ERROR: MethodError: no method matching MCMCMultiProposal(
    ::Tuple{RandomWalk{Float64, Tuple{Float64, Float64}, TDist{Float64}}},
    ::Vector{Int64},
)

Closest candidates are:
  MCMCMultiProposal(!Matched::P, ::R) where
    {P<:(Vector{<:BAT.MCMCProposal}), ...}

The equivalent vector input constructs successfully. The type parameter requires P<:Vector, while the zero-argument default is (RandomWalk(), HamiltonianMC()). That default also fails earlier when the optional AdvancedHMC backend is not loaded.

Please choose one public input representation, normalize accepted inputs, and make the zero-argument default usable without an unloaded optional backend. Validate non-empty proposals and picking-rule lengths. Add default, tuple/vector, and Categorical constructor tests.