New Element: Regular Block Model

[fwkoch]

In addition to name, description, metadata, axis_u/v/w, corner, and attributes, regular block model will have:

• block_size, a length-3 list of i, j, k size in doubles
• block_count, a length-3 list of number of blocks in i, j, k

Attributes are in a flat array, unwrapped in i then j then k. This should be explicitly documented.

[fwkoch]

For implementation details, please refer to the implementation and accompanying discussion in the example notebook: https://github.com/gmggroup/omf/blob/691420e3761c874139ca4127abae276204045111/notebooks/omf_cbi.ipynb?short_path=1aa1177#L93