Fixes rare issue with clearing of _value_cache

Deals with interesting issue with _value_cache where values cached in-between an "insert_timesteps" and a "get_values" could return incorrect results.
Merge pull request #49 from blazetopher/fix_valcache

Fixes rare issue with clearing of _value_cache
Allows append_parameter after coverage creation

Can now call append_parameter against a coverage at any time and the
parameter will be fully formed and functional.
The domain of the coverage will be applied to the parameter.
Adds test coverage for append_parameter.
Adds parameter exists check and test coverage.

Modifies test_append_parameter to use in_line writes.
Changes assertion method.
Adds assertion for duplicate parameter.