[scanbase] move _FOV to public.

/close #27
4 jobs for fix_27 in 13 minutes and 7 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Style
passed check_style #223891

00:00:17

 
  Test Library
passed test:python3.7_stretch_pyqt5 #223892

00:06:45

83.0%
passed test:python3.9_stretch_pyqt5 #223893

00:07:16

83.0%
 
  Test Notbooks
passed test:test-tomoscan-tutorials #223894

00:05:32