MIMO versions of PEM functions

[neveritt]

Description

MIMO versions of estimation methods for polynomial model structures are missing.

Methods to implement

• ARX
• ARMAX
• OE
• BJ
• FIR
• MORSM

Before the methods can be finalized, some external dependencies should be addressed.

External functionality needed

• Representation of polynomial matrices, separate package similar to Polynomials.jl?
• Filtering by polynomial matrix filters.
• LFD representation in ControlCore.jl