Fix assertions

[tobolar]

This PR especially

1. fixes senseless assertions like

   assert( body.r_0[2] - body.r_0[2] < tol ...);
   

   (so comparing the variable body.r_0[2] to itself).

   Fixing this, the assertion is triggered for tol=1e-4. So I additionally changed the intialization prismatic1.a(..., start=9-world.g) to have more precise value.

   Note: for ModelicaTest.MultiBody.Forces.Spring, this PR changes values of reference signals prismatic1.s and prismatic1.v over time. So regression check could be broken => reference signals shall be recalculated/re-generated.

   No such behaviour expected for ModelicaTest.MultiBody.Forces.Spring2.

2. Fixes missing absolute differences like abs(x - y) for assertions.

3. Separate tolerances to different physical quantities

4. SI units for tolerances

Refs #4193