enable lj/cubic for fix adapt

doc/src/fix_adapt.rst

@@ -184,6 +184,8 @@ formulas for the meaning of these parameters:
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/class2/coul/cut, lj/class2/coul/long <pair_class2>`                 | epsilon,sigma,coulombic_cutoff                   | type pairs  |
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
+| :doc:`lj/cubic <pair_lj_cubic>`                                              | epsilon,sigma                                    | type pairs  |
++------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/cut <pair_lj>`                                                      | epsilon,sigma                                    | type pairs  |
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/cut/coul/cut, lj/cut/coul/long, lj/cut/coul/msm <pair_lj_cut_coul>` | epsilon,sigma,coulombic_cutoff                   | type pairs  |

src/EXTRA-PAIR/pair_lj_cubic.cpp

@@ -347,3 +347,13 @@ double PairLJCubic::single(int /*i*/, int /*j*/, int itype, int jtype, double rs
 
   return factor_lj * philj;
 }
+
+/* ---------------------------------------------------------------------- */
+
+void *PairLJCubic::extract(const char *str, int &dim)
+{
+  dim = 2;
+  if (strcmp(str, "epsilon") == 0) return (void *) epsilon;
+  if (strcmp(str, "sigma") == 0) return (void *) sigma;
+  return nullptr;
+}

src/EXTRA-PAIR/pair_lj_cubic.h

@@ -38,6 +38,7 @@ class PairLJCubic : public Pair {
   void write_restart_settings(FILE *) override;
   void read_restart_settings(FILE *) override;
   double single(int, int, int, int, double, double, double, double &) override;
+  void *extract(const char *, int &) override;
 
  protected:
   double **cut, **cut_inner, **cut_inner_sq;

unittest/force-styles/tests/mol-pair-lj_cubic.yaml

@@ -18,7 +18,9 @@ pair_coeff: ! |
   3 3  0.02   3.2
   4 4  0.015  3.1
   5 5  0.015  3.1
-extract: ! ""
+extract: ! |
+  epsilon 2
+  sigma 2
 natoms: 29
 init_vdwl: 166.005266338502
 init_coul: 0