add caching to ModuleIO interface

[rvhonorato]

This PR introduces a caching system to the ModuleIO interface to hold values that should not be re-calculated across modules.

In this first implementation I only added an RMSD key but this can be further expanded to hold anything.

It works by adding a Cache to the ModuleIO interface that has full serialization support. It has a _store to hold the information and a _index as a helper for faster O(1) lookup (standard caching implementation).

Being part of the ModuleIO interface means it is accessible to all modules and save/load operations in this interface also applies to Cache.

I have added a new method update_cache to the interface that is used by the primitive module class to propagate the cache when the module export_io_models.

A relevant technical detail is that for proper typing and serializadion/de-serialization of the Cache we need to drop the jsonpickle approach to represent the ModuleIO interface in favor of a simple (but not human readable) pickle. I also observed that a lot of tests are using a file representation of this interface as arguments to test some behaviour - so I have added a fallback to the load method to handle this scenario. This is not ideal but its a sensible compromise.

No modules are using the cache yet!

This PR here just adds the system, I will refactor the modules to use the cache in another PR (related to #1530 and #1525)