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