Adds _append_external_links.

Adds function _append_external_links in order to append the HDF5 external links to the master metadata file.

Initialization of a SimplexCoverage with an empty ParameterDictionary not yet handled properly due to 1-D assumption upon initialization.
Removes improper implementation of init_master.
Adds test coverage for append_parameter.
Adds parameter exists check and test coverage.

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