Create group owner collections: e.g., Cells

[MicahGale]

The idea is that many convenience functions are needed when handling problems objects (e.g., a Cell). See: #11, #10, #19, #9, and expected failures in !11.

I propose that the be a Cells object that owns and manages the collection of cells. Proposed features:

1. Implements __getitem__ to allow accessing the list like a dictionary by cell number.
2. Implementing append to enforce object types, avoiding number collisions, etc.
3. Providing a cell_numbers generator that allows users to quickly check if a cell number is in use without the possibility of stale data.
4. Handling Cell-data cards and handling the switching of data cards to cell parameters. It will also store whether or not those data are on the cell or data cards.
5. Allow quick renumbering given an iter instance.
6. Surfaces should provide a lot of convenience functions to get all CZ surfaces, and find the smallest radius, etc.

This really is the central component to release-0.0.5

[MicahGale]

marked this issue as related to #11

[MicahGale]

marked this issue as related to #10

[MicahGale]

marked this issue as related to #19

[MicahGale]

marked this issue as related to #9

[MicahGale]

Items 4, and 5 are outside the scope of !13.

[MicahGale]

mentioned in commit 06952dc