Update ini pdf description

[joe-pitt]

The comments underneath [MCMC.PDF] in openghg_hbmcmc_input_template.ini could do with tidying. I had a look myself but got confused (e.g. in what circumstances is xprior a "dict of dict") so I think this needs doing by someone who understands these options better than me!

[brendan-m-murphy]

The "dict of dict" option is something @EricSaboya and @aliceramsden need for multi-sector models.

Maybe it would be useful to have a separate template/example ini file for that set-up. (Similarly, maybe we want a "basic" template for people getting started, vs. a PARIS etc. template with all the options.)