included MocoOutputExtremumGoal functionality

[nicos1993]

Brief summary of changes

Within MocoOutputGoal.h/.cpp I, with the help of Reed and Nick, have included a new goal, MocoOutputGoalExtremum, it is very similar to the original MocoOutputGoal. The goal permits the integration of only positive or negative values from a model output.

Testing I've completed

So far I have tested it locally on the walking_2d.cpp example and it is producing results as anticipated.

Looking for feedback on...

CHANGELOG.md (choose one)

I think I need to update the CHANGELOG_MOCO.md to include the 'MocoOutputGoalExtremum'.

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This change is 

[nicos1993]

I will update for the MocoContactImpulseTrackingGoal for now. Will update for this once we resolve everything (?).

[nickbianco]

@nicos1993 actually, you should add an entry to the Moco changelog. Then were good to go.

[nickbianco]

Great job @nicos1993!