MRCReader: assume file is little endian unless marked othewise
MRCReader: assume file is little endian unless marked othewise

Some MRC file writers don't follow the standard and fail to fill in the
endianness field.  So unless the file is marked as big endian, assume it is
little endian.

keithroe/vtkoptix,sumedhasingla/VTK,mspark93/VTK,keithroe/vtkoptix,sankhesh/VTK,SimVascular/VTK,SimVascular/VTK,gram526/VTK,sumedhasingla/VTK,mspark93/VTK,gram526/VTK,sankhesh/VTK,sankhesh/VTK,gram526/VTK,mspark93/VTK,mspark93/VTK,mspark93/VTK,keithroe/vtkoptix,sankhesh/VTK,gram526/VTK,gram526/VTK,keithroe/vtkoptix,mspark93/VTK,sumedhasingla/VTK,mspark93/VTK,gram526/VTK,sankhesh/VTK,gram526/VTK,sankhesh/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sankhesh/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,sumedhasingla/VTK,gram526/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,mspark93/VTK,keithroe/vtkoptix,SimVascular/VTK,sankhesh/VTK,SimVascular/VTK