add equilibrium search

[LukeLabrie]

This PR adds an equilibrium search method to the System class, along with a bit of refactoring on the solver methods, summarized below:

• Once max_delay is passed to the Solve method, it's then treated as a System attribute. This is so that it doesn't need to be passed as an argument so often
• Broke out the different solving cases, as well as other procedures pre & post integration, into different methods of the System class for readability/maintainability
• Bumped the required version of jitcdde due to fits fixed-step bug fixed by issue

This last point has broken the tests. For now, will wait to see if the bug fix is rolled into a new PyPI release. Otherwise, the test files will have to build jitcdde from its github repo