Optional validation step (and documentation improvements)

[fsoubelet]

This addresses #106

The main changes are:

• Version bump to 3.3.0
• Addition of a validate boolean argument in the reader, which defaults to True, enabling the user to skip the step.
• Addition of a validate boolean argument in the writer, which defaults to True, enabling the user to skip the step.
• Addition of tests (works similarly with/without the flag & expected warnings not issued when validation is skipped)
• Slight improvements to the documentation (few extra sphinx pluggins, admonitions, small examples of code, more info in some docstrings)

Worthy of noting:
This is a (very) significant speedup on large files. One can see a x4 speedup reading a typical LHC model twiss without validation, and a ~x11 speedup(!) on a 873M file (sliced fcc, 171k rows and 236 columns). See below a quick test.

Validation	No Validation

To be discussed:

• Should validation be on or off by default? Should that be different for reading / writing?
• Should we include changes for [Feature Request]: Check for madx-compatibility #104 in this PR?

On a side note, if someone know why the new documentation builds are weirdly expanding a lot in the index table of contents, I'd love to fix it too.