add variable lengths of steps in arc-length method #530
Description
- add feature
- add use of it in some test example, for instance... add a variable length step as another case in https://github.com/ONSAS/ONSAS.m/blob/master/examples/staticVonMisesTruss/onsasExample_staticVonMisesTruss.m
- include in the creatingmodels docs