Correct root=root -> root=0 in make_resume_file

[AdamOrmondroyd]

Previously, make_resume_file() in polychord.py (used only when the user provides their own initial samples) uses a try... except NameError block to identify whether MPI has been imported. However, the same block refers to root without it being defined, which will also throw a NameError.

Here I have corrected root=root to root=0, and updated the MPI check to set MPI=False if it can't be imported, and use an if instead of exception handling.

[williamjameshandley]

@maudformanek, I know that you have been using the make_resume_file() functionality, so this bug may have affected some of your runs (unless you fixed it locally).