Introduce enum `FileMode` for I/O file mode

[schnellerhase]

Currently the file mode in which an output file is opened is specified by a string.

with XDMFFile(mesh.comm, filename, "w", encoding=encoding) as file:
        file.write_mesh(mesh)

This should be altered to be enum based and back end independent, ref. #4177 (comment).

[michalhabera]

The design decision in XDMFFile is to follow standard approach from the C fopen, https://cppreference.com/c/io/fopen

However, given XDMFFile is considered to be soon replaced, not sure if it is worth breaking the API to change it.