fix discretizer packaging and optional testing
Previously the testing/importing 'continuum' would fail if sympy was not installed. Now we do the following: * add sympy as an optional dependency in 'extras_require' * force pytest to ignore tests in packages that have uninstalled dependencies by defining a hook in 'conftest.py' * use the 'class as a module' hack when importing 'continuum'. When sympy is not installed the continuum module will be replaced with an instance of 'ExtensionUnavailable' that will raise a runtime error on attribute access. * no warning is raised if sympy is not installed (it is an optional dependency).