add motors focus command and keyword

When the detector will be focused, we will end up with 3 motor positions that will become their nominal positions.

I suspect we'll store those positions somewhere (ics_config?) . 

Adding a few functionalities would be useful:

• xcu motors move focus command to go straight to that positions 
• xcu motors fullInit (fullInit is certaintly not the correct denomination), What I imply by that is doing an initCcd, homeCcd and move focus in one command
• motors focus keyword, that describe if the current position is nominal or not : Key('motorsPosition', Enum('focus', 'undef')