Bf gismo export

[FSchwar]

Overview

Fixed issues when exporting multipatches with None as field values.

Addressed issues

• multipatches with None in fields raised several errors

Checklists

• [ ] Documentations are up-to-date.
• [ ] Added example(s)
• [ ] Added test(s)

[FSchwar]

test_gismo fails in test_gismo_exportat line 84 - where the lines of the base_file and the temporary exported file are compared. I manually compared the files and it seems that only the order of the lines is off.

To be more precise, the definition of the boundaryConditions in the base_file is before the definition of Geometry, whereas in the temporary file (newly exported one), the definition of the boundary conditions is at the bottom of the xml file.

[jzwar]

There is still one issue that is unsolved in opinion at the moment. I think the reader will break if the spline ids are not contiguous, meaning if we only use splines [0,1,2,4,5] it will most likely break. This is because the the support matrix is referring to the splines in their global configuration, whereas the splines themselves are simply counted. There is two possible solutions to this issue. Either we introduce another column in the support matrix, where the global ids are matched a local id, this would have to be updated here and in the respective diffusion field. Second would not be a fix, but rather a warning, where the user is warned (in case a id_count!=id.

Please implement the latter, and I will make sure to update whenever this becomes an issue. Just make an elaborate warning, whenever this problem pops up

[jzwar]

Fixes #406