importing problems with new C functions?

Hello

@orbitfold @wkerzendorf 

I'm having trouble with import (this is with latest master branch of code):

```
In [1]: from tardis.tests import montecarlo_test_wrappers
WARNING: ConfigurationDefaultMissingWarning: Requested default configuration file /Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tardis.cfg is not a file. Cannot install default profile. If you are importing from source, this is expected. [tardis._astropy_init]
ERROR: ImportError: dlopen(/Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so, 2): Symbol not found: _compute_distance2electron
  Referenced from: /Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so
  Expected in: flat namespace
 in /Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so [IPython.core.interactiveshell]
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-155ec909d77a> in <module>()
----> 1 from tardis.tests import montecarlo_test_wrappers

ImportError: dlopen(/Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so, 2): Symbol not found: _compute_distance2electron
  Referenced from: /Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so
  Expected in: flat namespace
 in /Users/stuartsim/Tardis/tardis/build/lib.macosx-10.9-x86_64-2.7/tardis/tests/montecarlo_test_wrappers.so
```

This problem means that the test suite doesn't work for me just now. Since it's complaining about computedistance2electon, I wonder if it's something to do with having moved those functions to C? Any suggestions?