New Element: Regular Sub-Block Model

[fwkoch]

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

• parent_block_size = [i, j, k] doubles
• parent_block_count = [ni_p, nj_p, nk_p], integers
• sub_block_count = [ni_s, nj_s, nk_s], integers
• is_sub_blocked - exactly how this is stored needs final design. Likely stored in a way that may be easily compressed.

Attributes are in a flat array, unwrapped in i then j then k. If children are present, they are iterated over when they are encountered. 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#L189