Misc

Various improvements:

• Better print of delay systems (at least somewhat fixed now)
• Better error messages, indicating incorrect dimensions etc.
• Throw DimensionMismatch, etc, instead of just error. Possibly new error SampleTimeMismatch?
• Deal with freqresp, output format, exploit Hessenberg structure. Possibly drop nyquist (splitting into real and imaginary parts does perhaps not warrant its own function?).
• Use cis / expim for computing mappings to the unit circle.
• Perhaps add a type RationalLtiSystem (RLtiSystem) or at least isrational
• Should checking (approximate) equality of StateSpace etc. simply check that all the fields are equal? Or should the H_inf or H_2 norm of their difference be checked? Leaning towards checking the H_inf norm even if it is a bit more expensive. There is still a problem with DelayLtiSystems, but this is mostly relevant for tests where freqresp can be used.
• Export lag and perhaps also some sort of second_order_system?
• Set number of samples N for time-response of discrete-time system

Potential new features:

• FIR{T} <: RLTISystem{Discrete, T}
• New type NonlinearSystem?
• Print a delay system as a transfer function?
• Exact computation of H2 norms of delay systems according to https://people.kth.se/~eliasj/doc/delaylyap-2010-07-12.pdf