added schema and example for range Chaum-Pedersen proofs and eliminated placeholders

[eionblanc]

Issue

Fixes #292

Description

This pull request adds (not replaces) a schema detailing the use of range Chaum-Pedersen proofs instead of disjunctive and constant proofs and omitting the use of placeholder selections. For contests, it also amends the number_elected and votes_allowed conflation (mapping both to votes_allowed) and augments with the votes_allowed_per_selection to allow for selection encryptions of non-negative integers for the appropriate contests (e.g., cumulative voting).

Testing

This builds as expected.

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