=> faster computation of periodic boundary conditions.
+ Reorganize local_project.

Support the arguments nb_cells and offset.
Possibility of choosing the size of the triangles or (X-OR) the size of the unit cell.
Black formatting

Original commit for this change: 0b6c4d15

Improve structure of the code.
Better separation between the class definitions on one hand and the imports of the mesh and the subdomain data on the other hand.

Use of FenicsPart __init__ in Fenics2DRVE;
Define new functions for plotting the mesh and the subdomains and importing information about subdomains;
Split import of a mesh from a .xdmf and import of the subdomain data from the complementary .xdmf file;
Use of msh_conversion systematically when a conversion is needed.

https://docs.pytest.org/en/reorganize-docs/goodpractices.html

Only the components that must be equal to 0 owing to the microstructure symmetries and D_{122221} have been changed.