Only allow the lj key for Lennard-Jones input (#100)

As discussed in #99.

Author: Luthaf

doc/src/input/potentials.md

@@ -37,14 +37,14 @@ The Lennard-Jones potential is a classical potential for pair interactions
 expressed as: $$ V(x) = 4 \epsilon \left[\left(\frac{\sigma}{x}\right)^{12} -
 \left(\frac{\sigma}{x}\right)^6\right].$$
 
-The Lennard-Jones potential is defined using the `lj` or `lennardjones` key. The
+The Lennard-Jones potential is defined using the `lj` key. The
 parameters are `sigma` ($\sigma$) and `epsilon` ($\epsilon$), which should be
 provided as strings.
 
 ```toml
 [[pairs]]
 atoms = ["O", "O"]
-lennardjones = {sigma = "3.16 A", epsilon = "0.155 kcal/mol"}
+lj = {sigma = "3.16 A", epsilon = "0.155 kcal/mol"}
 ```
 
 ## Buckingham potential

src/input/src/interactions/pairs.rs

@@ -209,7 +209,7 @@ fn read_pair_potential(pair: &Table) -> Result<Box<PairPotential>> {
         match key {
             "null" => Ok(Box::new(try!(NullPotential::from_toml(table)))),
             "harmonic" => Ok(Box::new(try!(Harmonic::from_toml(table)))),
-            "lj" | "lennardjones" => Ok(Box::new(try!(LennardJones::from_toml(table)))),
+            "lj" => Ok(Box::new(try!(LennardJones::from_toml(table)))),
             "buckingham" => Ok(Box::new(try!(Buckingham::from_toml(table)))),
             "born" => Ok(Box::new(try!(BornMayerHuggins::from_toml(table)))),
             other => Err(