add quick access for Poisson ratio

src/data/INFO.json

@@ -48,7 +48,7 @@
     "E": "Young modulus, module of elasticity",
     "G": "Shear modulus (rigidity)",
     "K": "Bulk modulus",
-    "NU": "Poisson ratio ν",
+    "nu": "Poisson ratio ν",
     "Yield strength": {
       "R_s": "(aka R_e) it is detectable only when the material has the yield phenomenon (it does not exist for the cold drawn and the stainless steels)",
       "R_p": "it is detectable on cold drawn and stainless steel, in place of R_s",

src/data/example.json

@@ -25,7 +25,7 @@
     "E": [70, "GPa"],
     "G": [26, "GPa"],
     "K": [70, "GPa"],
-    "NU": 0.35,
+    "nu": 0.35,
     "Yield strength": {
       "R_p02": ["110-130", "MPa"]
     },

src/materials.py

@@ -192,10 +192,13 @@ def main():
                           help='list available material information')
   parser_get.add_argument('-r', '--rho', dest='const_collection',
                           action='append_const', const="rho",
-                          help='get density')
+                          help='get density rho')
   parser_get.add_argument('-E', '--E', dest='const_collection',
                           action='append_const', const="E",
-                          help='get Young\'s modulus, module of elasticity')
+                          help='get Young\'s modulus E, module of elasticity')
+  parser_get.add_argument('-n', '--nu', dest='const_collection',
+                          action='append_const', const="nu",
+                          help='get Poisson ratio nu')
   parser_get.add_argument('-R', '--R_p', dest='const_collection',
                           action='append_const', const="R_p",
                           help='get yield strength R_p')
@@ -228,6 +231,7 @@ def main():
       mat_index = {
         "rho": "Mechanical Properties/rho",
         "E": "Mechanical Properties/E",
+        "nu": "Mechanical Properties/nu",
         "R_p": "Mechanical Properties/Yield strength",
         "R_m": "Mechanical Properties/R_m"
       }