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.

The new condition format looks like this (for instance, for a parameter $p$ in the inhomogeneous part):

{
    "solver": "analytical",
    "state_variables": [
        "z"
    ],
    "initial_values": {
        "z": "1"
    },
    "condition": {
        "p==0": {
            "propagators": {
                "__P__z__z": "[...]"
            },
            "update_expressions": {
                "z": "[...]"
            }
        },
        "otherwise": {
            "propagators": {
                "__P__z__z": "[...]"
            },
            "update_expressions": {
                "z": "[...]"
            }
        }
    }
}

Replaces #89.

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.

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.