improve the getFE interface in Assembly #2668
enhance isNodal to allow more types of elemental variables #2668
better to move nodal variable check into the constructor #2668
Merge pull request #2670 from YaqiWang/isNodal_2668

Is nodal #2668
Merge commit 'ae9923b2504febc7b7e49cc1130b7ed5ea72fd50'