Update `backward3()` function

Updated backward3() function input arguments to include position vector
values.
Added `get_pos_vec.m` file to aid with the computation of position
vector values.
Complete `caldyn_leftleg()` function code

Complete code for `caldyn_left_leg()` function with force and moments
computation for all the body segments.
Fix matrix dimension issue in `get_pos_vec.m`, `newtontimeloop.m` and
`caldyn_leftleg.m`.
Reorder segments in `segments.m`.
Add test script to test `caldyn_leftleg()` function

Add test script to test `caldyn_leftleg()` function. Test results proved
that there is no Matlab errors. The numerical values of force and
moment still need to be verified with real data.
Fix minor bugs in `newtontimeloop.m` and `caldyn_leftleg.m`.