Fix Sympy >=1.5

[jjuch]

The functionality of Sympy's empty_array function has been altered internally in version >=1.5, which led to malfunction of the examples if installed with pypi (see #13 and #18 ) . A simple new empty_array function is added which does not depend on Sympy and works for the examples which are currently present in the module. Ignore the additional comment on line 152.

[sixpearls]

@jjuch thank you for identifying the underlying issue with the SymPy >= 1.5 compatibility. I need to decide about how I want to handle this. If you want, we can keep this PR open and, assuming something like this will be needed, I can give you instructions on how to make the fix in a way that fits with the rest of the code. You would definitely need to start from upstream and write production quality code (linted, no extraneous comments, tests, etc). Otherwise I'll close this and you're welcome to try submitting another PR to contribute to version 2.0.

[jjuch]

@sixpearls I agree with your strategy to get to the core of the problem related to SymPy's changes in version 1.5. As I'm building upon your code, I can only benefit from production quality code in this module. I would be glad to help out with SimuPy dev wherever I can. I await your instructions.

[jjuch]

@sixpearls This issue is still existing and needs to be fixed as soon as possible. With MemorylessSystem objects a similar issue arises in the state_equation setter. The state_equation is set to None by MemorylessSystem, but in the setter the grad function fails to handle the empty_array() object.