Add NumericVector::is_effectively_ghosted()

I only need this for PetscVector, for several layers of reasons at this
point, but I suppose it is the sort of thing that could be useful in
general and from user code, so we'll make it public and leave it in the
base class.

Author: roystgnr

include/numerics/numeric_vector.h

@@ -168,6 +168,19 @@ class NumericVector : public ReferenceCountedObject<NumericVector<T>>,
   bool is_effectively_serial() const
   { return (this->n_processors()==1) || (_type == SERIAL); }
 
+  /**
+   * \returns Whether the vector is effectively ghosted, i.e. whether
+   * vector operations require handling of ghosted coefficients.
+   *
+   * This is true for GHOSTED vectors in parallel, but in serial
+   * subclasses may return false here for GHOSTED vectors since every
+   * ghosted vector is effectively serial and thus may be able to take
+   * advantage of serial-specific code paths.
+   */
+
+  bool is_effectively_ghosted() const
+  { return (this->n_processors()!=1) && (_type == GHOSTED); }
+
   /**
    * \returns The type (SERIAL, PARALLEL, GHOSTED) of the vector.
    *