Equations for flexural tests: three- and four-point bending

Equations to obtain data from load in flexural/bending tests. 

Implemented in doi.org/10.5281/zenodo.7271867

NOTE: the normalisation of force-displacement to mean stress-strain [see Eqs. (49)-(51)] considers balanced beams/plates, i.e., the neutral axis of flexural/bending coincides with the geometric centroidal axis. For unbalanced structures, the limits of integration should be based on the neutral axis, calculated by the transformed-section method.