Add conditions to analytic integrators

[clinssen]

The current ODE-toolbox release detects conditions under which singularities can occur. This PR adds a feature that allows ODE-toolbox to generate separate solvers for each of these conditions.

Please see the documentation at: https://ode-toolbox-sandbox.readthedocs.io/en/latest

N.B. after merging this, the version number of ODE-toolbox should be bumped to 3.0.0, as this feature changes the external API.

Notes for reviewers:

• Split off testing requirements into a separate requirements-testing.txt.
• Sadly, the old 1.10.1 sympy version gets stuck on test_analytic_solver_integration.py and introduces numerical errors, so I've set the test to skip for older versions and added a line into the documentation.
• pycodestyle no longer ignores E303 (too many blank lines)

Replaces #89.