ActionSelection does not clean up after exception handling

[jgosmann]

When an exception was raised from within a spa.ActionSelection block, subsequent spa.ActionSelection blocks will produce an exception even though they are correct. Apparently the tracking of connections not enclosed in ifmax calls does not get reset and this lets the ActionSelection believe that there are still free floating connections.

Example exception:

---------------------------------------------------------------------------
SpaActionSelectionError                   Traceback (most recent call last)
<ipython-input-12-7720533943af> in <module>()
      5     with spa.ActionSelection():
      6         spa.ifmax(spa.dot(a, spa.sym.KEEP_INPUT), spa.sym.X >> b, (a * (~b).reinterpret()) >> c)
----> 7         spa.ifmax(spa.dot(a, spa.sym.KEEP_INPUT), spa.sym.X >> b, (a * (~b).reinterpret()) >> c)

~/Library/Python/3.7/lib/python/site-packages/nengo_spa/action_selection.py in __exit__(self, exc_type, exc_value, traceback)
     93         if exc_type is not None:
     94             return
---> 95         self._build()
     96 
     97     def _build(self):

~/Library/Python/3.7/lib/python/site-packages/nengo_spa/action_selection.py in _build(self)
     99             if len(RoutedConnection.free_floating) > 0:
    100                 raise SpaActionSelectionError(
--> 101                     "All actions in an action selection context must be part "
    102                     "of an ifmax call.")
    103         finally:

SpaActionSelectionError: All actions in an action selection context must be part of an ifmax call.