Generalize read_flat_file function

[xresende]

So that it can read a char-flat-encoded lattice from a std::string passed as argument. @fernandohds564 is trying to implement a code that serializes pyaccel objects using this work-around solution since apparently there are restriction on which swig objects can be pickled.

write_flat_file to a std::string has already been implemented. We need to export it to python (@afonsoharuo) and test it.

Exported as write_flat_file_to_string.