p0c setter gives an error

[giadarol]

Here an example:

In [7]: ps_part.p0c = 7000*1e9
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-6b6e194dff1d> in <module>()
----> 1 ps_part.p0c = 7000*1e9

~/Desktop/WeakStrongDevelopment/pysixtrack/pysixtrack/particles.py in p0c(self, p0c)
    282         new = self._f1(self.mass0, p0c)
    283         self._update_ref(*new)
--> 284         self._update_particles(*new)
    285 
    286     energy0 = property(lambda self: self._energy0)

~/Desktop/WeakStrongDevelopment/pysixtrack/pysixtrack/particles.py in _update_particles(self, mass0, beta0, gamma0, p0c, energy0)
    328             Energy = self.Energy
    329             Pc = self.Pc
--> 330             mratio = mass / mass0
    331             self._mratio = mratio
    332             self._chi = self._qratio / mratio

NameError: name 'mass' is not defined

[rdemaria]

Review whole logic of updates...