Added "m" to mes device. Updated Documentation and added mes test file.

tests/mes/subth.cir

@@ -0,0 +1,17 @@
+Mesfet subthreshold characteristics
+
+Vds 1 0
+vids 1 2 dc 0
+Vgs 3 0 dc 0
+
+z1 2 3 0 mesmod area=1.4
+
+.model mesmod nmf level=1 rd=46 rs=46 vt0=-1.3
++ lambda=0.03 alpha=3 beta=1.4e-3
+* z1 2 3 0 lev2 l=1u w=20u
+* .model lev2 nmf level=2 d=0.12u mu=0.23 vs=1.8e5
+*+ m=3.3 vto=-1.3 eta=1.82 lambda=0.044 sigma0=0.09
+*+ vsigma=0.1 vsigmat=0.9 rdi=46 rsi=46 delta=5
+*+ nd=2.1e23
+
+.end