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

- this commit should allow opening 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