Avoid errors with STATE variables without DERIVATE block

 * In case of state variables without ODEs or  SOLVE block we get
   errors like:

   V not really a STATE; Ie. No differential equation for it
   gNMDA not really a STATE; Ie. No differential equation for it

 * Not sure about the "desirable" fix here:

   - should we avoid declaring variables as STATE
   - use workaround of empty SOLVE block like this PR
   - have fix in neuron to avoid this as an error

Tagging @nrnhines for this to discuss.

izhi2007.mod

@@ -125,9 +125,11 @@ PROCEDURE useverbose() { : Create user-accessible function
 }
 
 
+PROCEDURE states() {}
 
 : Define neuron dynamics
 BREAKPOINT {
+  SOLVE states
   delta = t-t0 : Find time difference
 
   : Receptor dynamics -- the correct form is gAMPA = gAMPA*exp(-delta/tauAMPA), but this is 30% slower and, in the end, not really any more physiologically realistic