Expand Label to surface using patch info

Following up on issue https://github.com/mne-tools/mne-python/issues/1180. As general solution I propose adding the option to expand a label from a source space to the entire surface in the `Label` constructor. This might be useful for users that construct labels based on source space data themselves. If this is ok I will modify other functions to use this capability. An alternative would be to have  a separate `expand_label()` function and use it where appropriate.