Resolve "Add cuboid mesh generator"

Closes #46

See merge request mechanics-and-materials/ae108!38

Resolve "Add Timoshenko beam element"

Closes #35

See merge request mechanics-and-materials/ae108!33

- element_axis is already zero initialized

- avoids potential future name collisions

- avoids unnecessary copy

- avoids unnecessary includes in header

- reduces compile time for users

- eliminates unnecessary copies

- interprets matrix as vector

- avoids duplication of formulas

- disallows accidental calls