Added new cell type defined in cell_B.xml for use with parameters_B

This is an iafCell with an "activity" variable 0->1 which varies with
membrane potential, smmothed by tau1 (=50ms currently)
Gives a rough approximation of cell activity... Maybe closer to real
thing than v of an I&F cell
Merge branch 'master' of github.com:openworm/CElegansNeuroML

* 'master' of github.com:openworm/CElegansNeuroML:
  Adding image