Faster dataframe validation

[fsoubelet]

This addresses #108

The main changes are:

• Rework of the tfs.frame.validate function logic and removal of the old is_not_finite function and .applymap call.
• Addition of a check to detect lists / tuples elements with a custom vectorized function applied to each column.
• Use of the DataFrame.isna() with the right context options for the finite-values part of the checks.
• Inclusion of a methodology admonition in validate's docstring

The speedup, while not as nice as skipping validation like in #107, is nice. One can see a ~x5 speedup on a 873M file (sliced fcc, 171k rows and 236 columns). See below a quick test.

New Validation	Old Validation (v3.2.1)

[JoschD]

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