Create a "storage" sub-facility

[katyhuff]

We have discussed the issue of not being able to track/book-keep internal stocks of facilities (e.g. wet/dry storage). I propose a new module dev helper tool that will allow this. It will be a model subclass for which instances can easily be created without the use of dynamic loading/initialization.

Implementation:

• Inherit from Model class and satisfy entire concrete Model interface
• contain a single resource buffer
• provide add/remove resource methods that push/pull from the solitary resource buffer
• expose public methods that allow for querying/configuring the storage sub-facility's state (e.g. capacity, space, etc.) - these will essentially wrap some of ResourceBuff's methods.
• To move resources to/from a sub-facility storage, a facility will just create and execute transactions directly (not using messages or markets) with itself as the supplier/requester and the sub-storage as the counterpart.

Benefits:

• Allows for book-keeping of internal facility state
• doesn't require additional book-keeping infrastructure - relies on what we already have.
• Developers can choose whether or not the added effort/code is necessary. It is an opt-in thing.

Thoughts?

[katyhuff]

This issue came from https://github.com/cyclus/core/issues/148 . Go there for previous comments.

It was originally opened by @rwcarlsen .

[gonuke]

Not sure if this issue pre-dates the introduction of resource buffers, but some of the needs for this capability are not completely resolved by the existence of the resource buffers.