add extract() function to angle style cosine/squared

doc/src/fix_adapt.rst

@@ -365,11 +365,13 @@ all types from 1 to :math:`N`.  A leading asterisk means all types from
 If :doc:`angle_style hybrid <angle_hybrid>` is used, *astyle* should be a
 sub-style name. The angle styles that currently work with fix adapt are:
 
-+------------------------------------+----------+-------------+
-| :doc:`harmonic <angle_harmonic>`   | k,theta0 | type angles |
-+------------------------------------+----------+-------------+
-| :doc:`cosine <angle_cosine>`       | k        | type angles |
-+------------------------------------+----------+-------------+
++-----------------------------------------------+----------+-------------+
+| :doc:`harmonic <angle_harmonic>`              | k,theta0 | type angles |
++-----------------------------------------------+----------+-------------+
+| :doc:`cosine <angle_cosine>`                  | k        | type angles |
++-----------------------------------------------+----------+-------------+
+| :doc:`cosine/squared <angle_cosine_squared>`  | k,theta0 | type angles |
++-----------------------------------------------+----------+-------------+
 
 Note that internally, theta0 is stored in radians, so the variable
 this fix uses to reset theta0 needs to generate values in radians.

src/MOLECULE/angle_cosine_squared.cpp

@@ -291,3 +291,15 @@ void AngleCosineSquared::born_matrix(int type, int i1, int i2, int i3, double &d
   du2 = 2 * k[type];
   du = du2 * dcostheta;
 }
+
+/* ----------------------------------------------------------------------
+   return ptr to internal members upon request
+------------------------------------------------------------------------ */
+
+void *AngleCosineSquared::extract(const char *str, int &dim)
+{
+  dim = 1;
+  if (strcmp(str, "k") == 0) return (void *) k;
+  if (strcmp(str, "theta0") == 0) return (void *) theta0;
+  return nullptr;
+}

src/MOLECULE/angle_cosine_squared.h

@@ -36,6 +36,7 @@ class AngleCosineSquared : public Angle {
   void write_data(FILE *) override;
   double single(int, int, int, int) override;
   void born_matrix(int type, int i1, int i2, int i3, double &du, double &du2) override;
+  void *extract(const char *, int &) override;
 
  protected:
   double *k, *theta0;

unittest/force-styles/tests/angle-cosine_squared.yaml

@@ -16,7 +16,9 @@ angle_coeff: ! |
   3  50.0 120.0
   4 100.0 108.5
 equilibrium: 4 1.9216075064457567 1.9373154697137058 2.0943951023931953 1.8936822384138476
-extract: ! ""
+extract: |
+  k 1
+  theta0 1
 natoms: 29
 init_energy: 36.01162448576962
 init_stress: ! |2-