[Core] Allow structured mesh generation to use different mesh size in each direction

[jrubiogonzalez]

📝 Description This PR allows to specify different sizes in each direction for the structured mesh generation. Adding more flexibility.

🆕 Changelog Add option to use different size in each direction
Added test

[rubenzorrilla]

I would leave the name of the parameter as number_of_divisions and check if its a integer or an array (example: "number_of_divisions": [1, 1, 1] to be in line with variable values and tables.

Aside from that I see no problems.

On top of @roigcarlo's observation, I'd like to add that this is the approach we follow in similar cases (see for instance the assign scalar/vector processes).