AMReXGridReader: Use std::size_t or vtkIdType instead of long where necessary

• this commit fixes an integer overflow for files larger than 2GB on windows.
• it uses size_t/vtkIdType instead of long when doing stream operations like seekg, tellg. earlier, it passed long (32-bit in windows) to seekg. As a result, arrays beyond the 2GB offset could not be read into the buffer.
• see user report https://github.com/AMReX-Codes/amrex/issues/4167 and https://discourse.paraview.org/t/potential-length-overflow-for-large-data/15516

@patrick-oleary, @cory.quammen