build_transition_matrix called before applying selection

[Zsailer]

Calling build_transition matrix before calling apply_selection throws:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-10-0b8c154ec8f7> in <module>
      1 gpmsm.build_transition_matrix(    fixation_model=mccandlish, 
----> 2     population_size=100)

~/Documents/research/projects/pkgs/gpvolve/gpvolve/markovmodel.py in build_transition_matrix(self, fixation_model, **params)
     96 
     97         # Get fitnesses of all nodes.
---> 98         fitness1 = np.array([self.node[node]['fitness'] for node in nodepairs[0]])
     99         fitness2 = np.array([self.node[node]['fitness'] for node in nodepairs[1]])
    100 

~/Documents/research/projects/pkgs/gpvolve/gpvolve/markovmodel.py in <listcomp>(.0)
     96 
     97         # Get fitnesses of all nodes.
---> 98         fitness1 = np.array([self.node[node]['fitness'] for node in nodepairs[0]])
     99         fitness2 = np.array([self.node[node]['fitness'] for node in nodepairs[1]])
    100 

KeyError: 'fitness'

This is a cryptic error. Exception should point users to apply_selection method.