Fix package requires

The phasefield model requires the generation of a module by Cython. However, this file was not shipped with pypi and cython is not required by lbmpy.

lbmpy/phasefield/nphase_nestler.py

-import pyximport
+try:
+    import pyximport
+except ImportError:
+    print("pyximport could not be imported. nphase_nestler can only be used if Cython is installed")
+    raise
+
 import sympy as sp
 
 from lbmpy.creationfunctions import create_lb_update_rule