use user HOME dir for cache, config, models

[EricDeveaud]

related to #252

PR propose to have a $HOME/rfdiffusion that will hold all necessary files where write erpmission is required.
with this PR:

• inference config files location are seaerchd from $HOME/rfdiffusion/config/inference/
• IGSO3 are cahed to $HOME/rfdiffusion/schedules
• weight models are exepected in $HOME/rfdiffusion/models

furthermore the PR allows weight models directory search to be overwritten by RFD_MODELS environment variable.
by default check in $HOME/rfdiffusion/models and search $RFD_MODELS instead if is defined
(allow mutualisation of models on a shared cluster)
same for inference config files, if RFD_HYDRA_CFG is defined.

regards

Eric

[roccomoretti]

It might be better to use the user directory as a fall back. That is, instead of always writing to the user directory, only use the user directory if the installation directory isn't writable.

Additionally, I would probably recommend using a "hidden" directory (i.e. f"{USER_HOME}/.rfdiffusion") instead of the non-dotted one. (Creating a new top-level directory in a person's home directory that always shows up with a ls seems presumptuous. It also could be the case that they already have an rfdiffusion directory there, used for something else.