Allow the Gramps extension to export Gramps family trees automatically

[bartfeenstra]

Gramps comes with a command line that allows the export of family trees. The Gramps extension can then load these exports into an ancestry.

Definition of Done

• Instead of a file name, allow the Gramps configuration to alternatively use family tree names
• When loading a family tree by name, spawn a subprocess to make Gramps export the file to a temporary directory, from which it is then loaded as usual
• See https://github.com/bartfeenstra/ancestry/blob/main/bin/export for the Gramps command to export a family tree