enable lj/gromacs for fix adapt

doc/src/fix_adapt.rst

@@ -198,6 +198,8 @@ formulas for the meaning of these parameters:
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/expand <pair_lj_expand>`                                            | epsilon,sigma,delta                              | type pairs  |
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
+| :doc:`lj/lj/gromacs <pair_gromacs>`                                          | epsilon,sigma                                    | type pairs  |
++------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/mdf <pair_mdf>`                                                     | epsilon,sigma                                    | type pairs  |
 +------------------------------------------------------------------------------+--------------------------------------------------+-------------+
 | :doc:`lj/sf/dipole/sf <pair_dipole>`                                         | epsilon,sigma,scale                              | type pairs  |

src/EXTRA-PAIR/pair_lj_gromacs.cpp

@@ -432,3 +432,13 @@ double PairLJGromacs::single(int /*i*/, int /*j*/, int itype, int jtype, double
 
   return factor_lj * philj;
 }
+
+/* ---------------------------------------------------------------------- */
+
+void *PairLJGromacs::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_gromacs.h

@@ -39,6 +39,7 @@ class PairLJGromacs : public Pair {
   void write_data(FILE *) override;
   void write_data_all(FILE *) override;
   double single(int, int, int, int, double, double, double, double &) override;
+  void *extract(const char *, int &) override;
 
  protected:
   double cut_inner_global, cut_global;

unittest/force-styles/tests/mol-pair-lj_gromacs.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: 749.2511379269325
 init_coul: 0