Add convergence tolerance setting

[Technologicat]

Currently, pydgq considers an iteration complete once all bits of the double-precision 64-bit float agree.

While this is useful for very sensitive equations (e.g. those in undamped vibration problems), for other real-world use a convergence tolerance setting would be useful.

• Maybe options for absolute and relative.
• Maybe default to 1e-6.
• Let the user specify 0 to use the old behavior.