Explicitly import used libs from 'mathphys'.

This makes the code that uses siriuspy less restricted to some
dependencies, such as h5py.

siriuspy/siriuspy/util.py

@@ -11,7 +11,7 @@
 
 import epics as _epics
 
-from mathphys import beam_optics as _beam
+from mathphys.beam_optics import beam_rigidity as _beam_rigidity
 from siriuspy import envars as _envars
 
 
@@ -134,7 +134,7 @@ def configure_log_file(stream=None, filename=None, debug=False):
 
 def beam_rigidity(energy):
     """Return beam rigidity, beta amd game, given its energy [GeV]."""
-    brho, _, beta, gamma, _ = _beam.beam_rigidity(energy=energy)
+    brho, _, beta, gamma, _ = _beam_rigidity(energy=energy)
     return brho, beta, gamma