initial import of adapted gREM code by David Stelter and Edyta Malolepsza

The following changes were made:
- the modifications to compute pressure were transferred to a derived class compute pressure/grem
- fix scaleforce was renamed to fix grem
- identifying the grem fix was simplified as fix grem passes an additional argument to compute pressure/grem
- dead code was removed in both files
- checking of arguments was tightened

src/compute_pressure.h

@@ -28,9 +28,9 @@ class ComputePressure : public Compute {
  public:
   ComputePressure(class LAMMPS *, int, char **);
   virtual ~ComputePressure();
-  void init();
-  double compute_scalar();
-  void compute_vector();
+  virtual void init();
+  virtual double compute_scalar();
+  virtual void compute_vector();
   void reset_extra_compute_fix(const char *);
 
  protected: