FIX: _nested_update method in config.py fails to add new keys

[jerdra]

This relates to applying the global configuration settings to each FileSpec object. When a key is missing from a FileSpec configuration that is present in the global the intended behaviour is that the key defined in global gets inserted into the FileSpec configuration.

Currently this operation does not insert keys if missing i.e if:

bids_map:
   sub:
     regex: ABC

is defined under globals and the FileSpec bids_map is missing sub, then we should expect that sub gets inserted into a FileSpec's bids_map. But currently this is not the case.

The fix should be implemented in _nested_update in niviz/config.py to add keys when missing. In addition, only keys intended to be used in the FileSpec definition should be used, in which case we may need a FileSpec context (key) under global so we know which keys can be applied to FileSpec items.