Uncomment first derivative optimization (#3356)

This was commented due to a now fixed libmes/fparser bug
git Deprecate 'read_legacy in SolutionUserObject
(closes #3355)
Merge pull request #3484 from dschwen/chparsed_3356

Uncomment first derivative optimization
Merge pull request #3482 from aeslaughter/xda-3355

Deprecate 'read_legacy' in SolutionUserObject