Trac #28458: Housekeeping for hypergeometric motives

This ticket includes some housekeeping for
`sage.modular.hypergeometric_motive.HypergeometricData`.
* Reordered methods into clearer categories (internals,
parameters/invariants, operations, L-functions).
* Added some utility methods: `alpha`, `beta`, `hodge_polygon_function`,
`hodge_polygon_vertices`.
* We now cache some of the parameters at creation time, to reduce
overhead when computing many Euler factors.
* Some simple but effective optimizations to `padic_H_value`. Most
significantly, we eliminate redundant calls to `padic_gauss_sum`.

In addition, we make some tweaks to
`sage.rings.padics.padic_generic_element.gamma`, including optimizing
the working precision.

URL: https://trac.sagemath.org/28458
Reported by: kedlaya
Ticket author(s): Kiran Kedlaya
Reviewer(s): Frédéric Chapoton

Author: Release Manager