Linear equations support

This adds support for linear equations using the sympy method (works only for diagonalizable coefficient matrices but allows for non-constant parts in the rest (non-multiplicative) of the equations). This is not complete yet (we want to add something more close to the Brian1 system, working for non-diagonalizable matrices) and needs more testing, but so does the whole state updater system. Therefore I think it's ready to merge as it is. Closes #13