Deprecate the Parameter attribute pickle_default_value?

[maximlt]

pickle_default_value is defined as follows:

        pickle_default_value: whether the default value should be
        pickled. Usually, you would want the default value to be pickled,
        but there are rare cases where that would not be the case (e.g.
        for file search paths that are specific to a certain system).

and set to False (True by default) in resolve_path.search_paths and resolve_path.path_to_file. It has been in Param since at least its first commit on Github.

However, Param doesn't make any direct use of it internally, i.e. pickling a Parameterized object will not take into account its value, users have to write custom. The only usage I could find of it on Github is in topographica (https://github.com/search?q=repo%3Aioam%2Ftopographica%20pickle_default_value&type=code). There are no references to this Parameter attribute in Param's own issues or PRs (https://github.com/search?q=repo%3Aholoviz%2Fparam+pickle_default_value&type=issues).

I would suggest deprecating this Parameter attribute.