Removed the FieldCollection alias

Summary:
The FieldCollection<Dim_t DimS, Dim_t DimM, bool isGlobal was
confusing and ultimately unnecessary. It is more readable to
explicitly create GlobalFieldCollection<Dim_t DimS, Dim_t DimM> or
LocalFieldCollection<Dim_t DimS, Dim_t DimM> as needed. Discussion
of this is in Z75.

Test Plan: CTests pass in clang-5 and gcc-7

Reviewers: pastewka

Reviewed By: pastewka

Differential Revision: https://c4science.ch/D162