Convert to load_from_config() handling

Convert to load_from_config() handling

Rather than juggling independent configurations, consolidate them
under ScanConfig in the style of pyafm.storage.  This makes it easy to
load a full scanning config from ~/.config/unfold-default.yaml, which
is a plain-text file that can easily be tweaked by hand.

To generate an initial unfold-default.yaml file, try something like:

  >>> import unfold_protein.storage as storage
  >>> config = storage.get_default_config()
  >>> storage.save_scan_config(config=config)'

which uses defaults from unfold_protein.config as well as the default
AFM configuration from pyafm.storage.load_config().