Fixes incorrect usage of ParameterDictionary
Implements efficient persistence of SparseConstantValue

Implements a new *Storage class "SparsePersistedStorage" facilitating efficient persistence of SparseConstantValue.

Modifications to SparseConstantValue and test classes as necessary; including adding test_sparse_constant_value_interop
Adds configuration of logging for coverage model tests
Adds __getitem__ support to ParameterDictionary

Added purely for convenience when operating at the command-line.

__setitem__ is NOT allowed - use .add_context instead.