#2 Assertion for negative mue

[Michael M. Tiller]

The frictional components should contain assertions for negative mu (friction coefficient) values. This will
help detect when bad friction data has been entered. Furthermore, since the current interpolation system
extrapolates, it is possible (likely?) that a table indicating diminishing friction as a function of speed will
eventually lead to "negative friction" which will cause the system dynamics to behave very strangely.