This PR updates themateqn module to allow the use of SciPy linear algebra functions as a replacement for Slycot functions. Among other things, this allows the use of thelqr command without having to install Slycot.

I implemented this by changing all of themateqn functions (lyap,care,dare, etc) to accept atmethod keyword that can either bescipy orslycot. The default value formethod isslycot if it is installed, otherwisescipy, so there is no change in behavior if you have Slycot.

Other changes:

• Updatedlqr to callcare rather than replicating the code and cleaned up a lot of the code inmateqn to be more Pythonic.
• Modified theslycot_check function so that it stores the result of checking whetherslycot can be imported rather than trying to import every time the function is called.
• Implemented a function for checking size and properties of matrices passed tomateqn functions (mateqn._check_shape) that provides more consistent errors (this also required updating some of the unit tests).
• Modifieddare to solve the generalized Sylvester equation usingslycot.sg02ad rather than usingslycot.sg02md to solve the simplified form of the discrete algebraic Riccati equation because there seems to be an error in the way thatstabilizing is handled insg02md (it returns the closed loop eigenvalues in the wrong order). (I'm working on generating a concrete counterexample for posting to Slycot.)