Consolidated MIPSparseField and MIPDenseField into MIPField.
  Created MIPSparseField and MIPDenseField as empty classes inheriting MIPField for code compatibility.
Re-implemented resampling code.
Implemented precise power-of-2 resampling code for makeMIP().
Field<T> : Renamed classType() -> staticClassType() and introduced a virtual classType().
  Matches the signature of staticClassName() vs. className().
Added FieldRes::voxelCount() for an idea how many voxels are actually in use.
In PatternMatch.{h,cpp}, made both '-' and '^' work as negating qualifiers.
Updated unit tests for MIP fields. Removed files that were consolidated into MIPField.h.
Fixing compiler warning per Rob P.