mixd dual

[j042]

extends mixd export to properly set dual information, if needed. This is required for some MUM, e.g., VRDMUM. For easier use, this PR also adds to_subelements() to Faces and Volumes.

@felipegonzalezco - as discussed

fixes #159

[j042]

@felipegonzalezco is there a case where dual and mrng have different values?

[clemens-fricke]

I can not really review this due to me not knowing how the dual file/definition is implemented/setup for mixd.

[j042]

I can not really review this due to me not knowing how the dual file/definition is implemented/setup for mixd.

dual information provides element id number of neighboring element at that subelement. A subelement without a neighboring element would be a boundary element, which won't be modified.
Here, we find this interface/connectivity (that's at least what we call it in splinepy), by first extracting subelement centers and creating k-d tree. Then, we call radius_query on the centers themselves. Looping over the results and checking the len of the query hit ensures the uniqueness of the interface. Alternatively, if you don't need to check those and you have funi, you could just loop over the inverse instead and that will be a much faster query

[felipegonzalezco]

@felipegonzalezco is there a case where dual and mrng have different values?

Sure, in cases where you have immersed boundaries, mrng overwrites the dual info. That's why both files are required in XNS

[j042]

@felipegonzalezco is there a case where dual and mrng have different values?

Sure, in cases where you have immersed boundaries, mrng overwrites the dual info. That's why both files are required in XNS

Ok, that means we shouldn't write over the mrng array. I will update this