Document the data representation model in IHM

[tomgoddard]

It would be useful to describe the model representation used by IHM in a precise way but in higher level terms than the current IHM documentation

https://github.com/salilab/mmcif/blob/master/dictionary_documentation/documentation.md

Such a description will help assess the generality of the model representation. I'll attach an attempt at such a description. This probably belongs in the above documentation.md but I didn't want to tamper with that file unless others agree.

[tomgoddard]

Here's a rough cut at the level of model description useful for writing software that visualizes this data.

ihm_data_definition.txt