Improve value conversion when writing to waveform value

Use numpy.require to ensure value is of the correct format.  Also improve
validation of written value.

This addresses Jira CGP-182

Author: Araneidae

python/softioc/device.py

@@ -326,7 +326,12 @@ def set(self, value,
             # Note that the trailing null is needed to work around problems with
             # some clients.
             value = numpy.fromstring(value + '\0', dtype = 'uint8')
+
+        value = numpy.require(value, dtype = self.dtype)
         self._value = (+value, severity, alarm, timestamp)
+        if value.shape == ():  value.shape = (1,)
+        assert value.ndim == 1, 'Can\'t write multidimensional arrays'
+
         self.trigger()