Setting exponent to 1 in MocoOutputGoal can return negative values

[carmichaelong]

https://github.com/opensim-org/opensim-core/blob/c6dc2a5913df2cb7e7a72da1ebb162fda9eaa022/OpenSim/Moco/MocoGoal/MocoOutputGoal.cpp#L84

If the exponent is set to 1, the integrand can return negative values. Looks like all other values for the exponent will return positive values.

[nickbianco]

We can't use the abs() function here since it is non-smooth (it's fine to use abs() in the third branch of the if-statement since the resulting value is raised some a power > 1, making it smooth).

We should improve the documentation for setExponent() to make it clear what happens in each branch of the if-statement.