working poc for numexpr
Adds support for Dictable types in lists/tuples
Adds and integrates *Expression classes

Adds AbstractExpression, NumexprExpression, and PythonExpression classes in coverage_model.parameter_expressions module
Integrates the *Expression classes into the ParameterFunctionType/Value classes.
Adds example "sbe37im_samplecov" showcasing usage
Adds coverage_model.utils.express_slice

utils.express_slice replaces any None components of a slice_ with real values

For example - given a total domain of (20,10):
(slice(None,None,10),slice(3,None,None)) ==> (slice(0,20,10),slice(3,10,1))
Adds value caching to SimplexCoverage

Adds capability to cache the returns of calls to get_parameter_value to improve repeated call performance
Adds memoized_values to PFV
Allows retrieval of ind params & fun maps

Adds get_function_map and get_independent_parameters functions to ParameterFunctionType.

get_function_map returns a nested dictionary for all *Expressions required by the ParameterFunctionType's expression

get_independent_parameters returns a list of the independent parameters required by the ParameterFunctionType's expression
Adds pydot dependency; get/write_dependency_graph

Adds functions (get_dependency_graph and write_dependency_graph) for retrieving the graphviz DOT style digraph for the ParameterFunctionType's *Expression
Adds get_dependent_parameters()

Adds get_dependent_parameters function - the set of dependent functions this ParameterFunctionType's *Expression requires
Adds ParameterFunctionValidator; adds networkx dep

Adds ParameterFunctionValidator class - combines parameters from disparate locations.  PFV.validate() function allows validation that specific outputs can be provided based on the available parameters

Adds networks dependency.  Utilizes pydot for writing .dot files when requested - writing does NOT require the graphviz package!!
Refactors *Expression to *Function; adds value_encoding

Renames Abstract, Python, and NumexprExpression to Abstract, Python, and NumexprFunction

Adds support for value_encoding to ParemeterFunctionType
Adds cursory handling for function exceptions

Adds catch for exceptions raised during execution of ParameterFunctions - current handling is to return a properly-sized array full of fill_values.

Also adds replacement of NaN's with fill_value in returned array
Adds tests for ParameterFunction framework
Adds additional tests & improves existing ones
Adds some tests for ParameterFunctionValidator
Moves ParameterFunctionException; more tests

Moves ParameterFunctionException to parameter_functions module

Adds try/except to ParameterFunctionValidator.validate so that a ParameterFunctionException is raised

Adds additional tests for ParameterFunctionValidator
Adds param_map argument to PythonFunction
Tweaks to NumexprFunction; dict support in Dictable

Also changes the sbe37im_samplecov example to use the parameters from test_parameter_functions to prevent duplicate code.
Applies param_map at evaluation time
Updates reference for Gibbs Seawater

Updates GSW reference to Luke's fork because the version from PyPi doesn't include the required data files