Added DA syn, test synapse

data/neurons/mechanisms/DASyn.mod

@@ -0,0 +1,38 @@
+NEURON {
+    POINT_PROCESS DASyn
+    RANGE quanta, tau, open
+}
+UNITS {
+    (mM) = (milli / liter)
+}
+
+PARAMETER {
+    quanta = 1e-4 (mM/ms)
+    tau = 10 (ms)
+}
+
+INITIAL {
+    open = 0
+}
+
+STATE { 
+    open (1)
+}
+
+BREAKPOINT {SOLVE state METHOD cnexp}
+
+DERIVATIVE state { 
+    open' = -open/tau
+}
+
+
+: 33000 dopamine molecules per vesicle : Omiatek, D., Bressler, A.,
+: Cans, AS. et al. The real catecholamine content of secretory vesicles
+: in the CNS revealed by electrochemical cytometry. Sci Rep 3, 1447
+: (2013). https://doi.org/10.1038/srep01447
+
+
+
+NET_RECEIVE(weight) {
+    open = open + weight
+}