Ensure proper data types for command line arguments to fix #601

[haraldkl]

The command line arguments also have to conform with the ProjectSettings data types. With these changes they are also passed through the converter that is also used for the MD meta data. I am not quite sure if this is the proper way to go about it, but it resolves #601 for me.

[haraldkl]

Looks like this fails the test "test_extra_mods_empty":

string_list = [''], name = 'extra_mods', sep = ':'

    def _parse_to_dict(string_list: List[str], name: str, sep: str) -> Dict[str, str]:
        """Parse a list of strings of form "key = value" into a dict
    
        Parameters
        ----------
        string_list : List[str]
            List of strings to parse
        name : str
            Name in parent settings object, only used for error message
        sep: str
            Separator between key and value
    
        """
    
        result = {}
        for string in string_list:
            try:
>               key, value = string.split(sep, 1)
E               ValueError: not enough values to unpack (expected 2, got 1)

/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/settings.py:430: ValueError

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/home/runner/work/ford/ford/test/test_md_inputs.py:22: in run_ford
    ford.run()
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/__init__.py:486: in run
    proj_data, proj_docs = initialize()
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/__init__.py:137: in initialize
    proj_docs, proj_data = load_settings(proj_docs, directory, args.project_file.name)
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/__init__.py:316: in load_settings
    proj_data, proj_docs = load_markdown_settings(directory, proj_docs, filename)
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/settings.py:312: in load_markdown_settings
    settings = convert_types_from_metapreprocessor(ProjectSettings, settings, filename)
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/settings.py:386: in convert_types_from_metapreprocessor
    settings[key] = convert_setting(default_type, key, value)
/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/ford/settings.py:362: in convert_setting
    return _parse_to_dict(value, name=key, sep=sep)
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[ZedThree]

Thanks @haraldkl ! There was just a typo in the call to _parse_to_dict, I've fixed that and added a test to cover --external_links explicitly

[haraldkl]

Thanks a lot for the quick look into this, the fixes and merging!