Adds support for fixed length and constant strings

Updates the ConstantType parameter type so that it can support constant strings.  Also makes some improvements to efficiency and flexibility.

Ensures support for fixed-length strings (via QuantityType)

Updates related tests appropriately
Fixes bug in validation of string ParamTypes
Added check for strings for constant value
Fixes issue with CategoryValue & fill_value
Fixes issue with ConstantType.is_valid_value
Updates examples
Adds ConstantRangeType and ConstantRangeValue

Adds ConstantRangeType class and ConstantRangeValue class.

Updates examples to include exemplar usage

Adds ConstantRangeType to coverage_model/__init__.py
Extends flexibility of setting CategoryValue

CategoryValue can be set by assigning either one of the keys OR values for the code set.

CategoryType.is_valid_value filled in and utilized to ensure only valid categories are assigned.
Fixes CategoryValue bug w/ InMemoryStorage

Fixes minor bug with CategoryValue when using with InMemoryStorage

Adds cattype / catval to coverage_model.test.examples
Fixes bug in ConstantRangeValue

Allows fill_value of None to operate correctly
Fixes issue with categories after json encoding

Json does not allow integer keys in dictionaries.  Must override _fromdict and reset the keys in categories to the appropriate type.
Updates ParameterContext ATTRS

Adds display_name
Renames references_list --> references
Overrides _todict in CategoryType
Overrides _todict function in CategoryType to stringify the keys of the categories dict - allows interoperation with json and couchdb
Modifies handling of _template_attrs

Sets the value of each member of _template_attrs to None after generating the attribute.  The keys are left as they are utilized at higher levels of the model (i.e. from ParameterContext)
Fixes storage_encoding for CategoryType

Also applies key_fill_value argument to fill_value attribute
Fixes bug with setting default fill_value; update tests

Improves leveraging of numpy.dtype.kind and np.finfo and np.iinfo functions to result in default fill_values that are more compliant with the requested value_encoding

Updates tests, including adding testing of ConstantRangeType
Fixes fill_value setting for CategoryRangeType
Fixes ConstantRangeValue when empty
Adds checking for numpy scalars to utils.hash_any