Defer import of flfm.batch in cli.py (and maybe all other imports of flfm.reconstruction/io)

[jamienoss]

flfm.main.cli.py.main has a backend arg, however, the backend will be imported by flfm.batch before it the cli gets to dynamically pick and reload. In this interval it will load the default (currently pytorch).

[jamienoss]

This also makes me wonder if the CLI --backend arg is worth it or really needed since the FLFM_BACKEND setting can be set as an env var before invoking python.