fermipy should accept pathlib.Path objects

[HealthyPear]

also easy to add checks on existence and enforce directory structure within an analysis

currently, gta.GTAnalysis() fails even starting with

---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
Cell In[14], [line 1](vscode-notebook-cell:?execution_count=14&line=1)
----> [1](vscode-notebook-cell:?execution_count=14&line=1) gta = GTAnalysis(paths.data / "Fermi_LAT" / "fermipy_config_baseline.yml", logging={'verbosity' : 5})
      2 gta.setup()

File ~/Data/projects/W51_FermiLAT_baseline/.snakemake/conda/91ff16a64e21ea2415c95ceb35b74ee7_/lib/python3.11/site-packages/fermipy/gtanalysis.py:294, in GTAnalysis.__init__(self, config, roi, **kwargs)
    291 validate = kwargs.pop('validate', True)
    292 self._loglevel = kwargs.pop('loglevel', logging.INFO)
--> [294](https://file+.vscode-resource.vscode-cdn.net/Users/michele/Data/projects/W51_FermiLAT_baseline/src/scripts/~/Data/projects/W51_FermiLAT_baseline/.snakemake/conda/91ff16a64e21ea2415c95ceb35b74ee7_/lib/python3.11/site-packages/fermipy/gtanalysis.py:294) super(GTAnalysis, self).__init__(config, validate=validate,
    295                                  **kwargs)
    297 self._projtype = self.config['binning']['projtype']
    298 self._tmin = self.config['selection']['tmin']

File ~/Data/projects/W51_FermiLAT_baseline/.snakemake/conda/91ff16a64e21ea2415c95ceb35b74ee7_/lib/python3.11/site-packages/fermipy/config.py:219, in Configurable.__init__(self, config, **kwargs)
    217     raise Exception('Invalid path to configuration file: %s' % config)
    218 else:
--> [219](https://file+.vscode-resource.vscode-cdn.net/Users/michele/Data/projects/W51_FermiLAT_baseline/src/scripts/~/Data/projects/W51_FermiLAT_baseline/.snakemake/conda/91ff16a64e21ea2415c95ceb35b74ee7_/lib/python3.11/site-packages/fermipy/config.py:219)     raise Exception('Invalid config argument.')
    221 self.configure(config_dict, **kwargs)
    223 if self.configdir and 'fileio' in self.config and \
    224         self.config['fileio']['outdir'] is None:

Exception: Invalid config argument.

unless using str(path_to_config)

The error itself is not informative for the user (see #617) and might lead to people opening issues for every small mistake.