header_type hard-coding

[DanW97]

I am wondering about the rationale behind hard-coding header_type to UInt64 and if it is possible to detect the type from the file instead? I've ran into this a few times whilst trying to work with vtk files produced by pyvista for example.

[ranocha]

If you would like to improve this, please feel free to make a PR and add some tests.

[DanW97]

I should have some availability in the new year to work on this.