electromagnet: set busy status before returning from write_target

+ add meaning to magnetic field

cfg/emagnet_cfg.py

@@ -10,6 +10,7 @@ IO('io_mot', uri='serial:///dev/ttyS2?baudrate=115200')
 Mod('mf',
     'frappy_psi.electromagnet.Magnet',
     'magnetic field',
+    meaning=('magneticfield', 10),
     io='io_mf')
 
 Mod('mot',

frappy_psi/electromagnet.py

@@ -194,6 +194,7 @@ class Magnet(HasIO, Drivable):
 
     def write_target(self, target):
         self._busy = True
+        self.status = BUSY, 'changed target'
         self.setFastPoll(True)
         # self.write_main_switch(True)
         self.io.send_write_message(('setpoint', 'Magnetcurrent'), target / self.scale)