obey numexpr 2.9 (?) expression sanity check rules

otherwise numexpr raises an error "expression xxx has forbidden control characters"

postpic/helper.py

@@ -946,7 +946,7 @@ def _linear_phase(field, dx, phi0=0.0):
 
  # calculate linear phase
  # arg = sum([dx[i]*mesh[i] for i in dx.keys()])
- arg_expr = '+'.join('({}*k{})'.format(repr(v), i) for i, v in dx.items())
+ arg_expr = '+'.join('({:}*k{})'.format(v, i) for i, v in dx.items())
 
  if transform_state is True:
  exp_ikdx_expr = 'exp(1j * ({arg} + phi0))'.format(arg=arg_expr)
@@ -1057,7 +1057,7 @@ def _kspace_propagate_generator(kspace, dt, moving_window_vect=None,
  # m = kspace.matrix.copy()
  # m[sum(k*dx for k, dx in zip(kspace.meshgrid(), moving_window_vect)) < 0.0] = 0.0
  # kspace = kspace.replace_data(m)
- arg_expr = '+'.join('({}*k{})'.format(repr(v), i)
+ arg_expr = '+'.join('({:}*k{})'.format(v, i)
  for i, v
  in enumerate(moving_window_vect))
  numexpr_vars = dict(kspace=kspace)