WIP: External package for matrix equations

[olof3]

This changes to an external package for matrix equations that I've been trying to put together here https://github.com/olof3/ControlMatrixEquations.jl. See #303 for some benchmarks of the linear solvers and some thoughts on why that package seems like a better way forward (at least for the short term) than going for MatrixEquations. Some things work better than in MatrixEquations, other things work less well (for example informing about numerical problems).

So would be great with some people using that package and finding problems (or even contributing). Hopefully some kind of joint effort with MatrixEquatons could be made. Regardless of what we decide on I think it would be good if the package became a part of the JuliaControl organization.

Some issues that gets fixed by this:

With the better performance of lyapd it is possible to compute norms of systems with > 150 states (fixes #300).

Problems with Ricatti equations that gets fixed: #281, #297, #440

[baggepinnen]

Any updated thoughts on this? This is a nonbreaking change?