Misc

Various improvements:

• Should checking equality of StateSpace etc. simply check that all the fields are equal?
• Better print of delay systems
• 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 compute mappings to the unit circle.
• Perhaps add a type RationalLtiSystem or at least isrational

Potential new features:

• 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