Add primitive caching

[csala]

The primitive JSON files should be looked for in at least two places:

• the package primitives directory
• a custom user directory, such as ./mlblocks_primitives

There should also be a method to add a custom folder where primitives should be taken from.

There should also be a dict somewhere in the package where the loaded primitives would be stored "in memory" to avoid having to load them from disk more than once.

[csala]

Most of this issue has already been implemented, but the caching is not in place yet.

The behavior will be:

• MLBlock will store the loaded primitive JSON as a dict in memory, to avoid having to read it again if the same primitive is used again.
• Both MLBlock and MLPipeline will accept a new argument cache, which will default to True.
• If this is True, the dict will be looked up and used. If it is set to False, it will always re-load the JSON from disk, and overwrite the previously stored dict.