Support for voltage and Ca dependent channel in NeuroML2 reader

- array_eval_component() reimplements neuroml expression evaluation
                         using numpy vectorization. This adds
			 dependency on pint for unit handling.
- Numerous other fixes and cleanups.

biophysics/HHGate.cpp

@@ -26,7 +26,7 @@ const Cinfo* HHGate::initCinfo()
         "Alternatively uses linear interpolation."
         "The range of the double is predefined based on knowledge of"
         "voltage or conc ranges, and the granularity is specified by"
-        "the xmin, xmax, and dV fields.",
+        "the xmin, xmax, and xdivs fields.",
         &HHGate::lookupA);
     static ReadOnlyLookupValueFinfo<HHGate, double, double> B(
         "B",