Sympy matrix exponential performance

[clinssen]

Sympy releases after 1.4 introduce a regression in runtime performance when computing matrix exponentials. This issue is being tracked at sympy/sympy#19887.

One solution is to use sympy==1.4, but this version is quite old (and see #77 (comment)). A workaround was added in #70, which considerably reduces the size of the matrices going into expm(), avoiding the performance issue. If performance issues still arise for particular, high-dimensional dynamical systems, try using sympy==1.4 or check sympy/sympy#19887.