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.
Merge pull request #45 from oceanzus/append_param

Append parameter