Remove surface function class.

We will just use the string-Eigen::Vector mapping in the top-level Meddle object

Author: Roberto Di Remigio

doc/code-reference/helper-classes.rst

@@ -45,14 +45,6 @@ Solvent
    :protected-members:
    :private-members:
 
-SurfaceFunction
----------------
-.. doxygenclass:: SurfaceFunction
-   :project: PCMSolver
-   :members:
-   :protected-members:
-   :private-members:
-
 Symmetry
 --------
 .. doxygenclass:: Symmetry

doc/users/interfacing.rst

@@ -11,7 +11,12 @@ If your program is written in C/C++, head over to :ref:`C-example`
 How PCMSolver handles potentials and charges: surface functions
 ---------------------------------------------------------------
 
-:cpp:class:`SurfaceFunction`
+Electrostatic potential vectors and the corresponding apparent surface
+charge vectors are handled internally as `surface functions`.
+The actual values are stored into Eigen vectors and saved into a
+map. The mapping is between the name of the surface function, given by
+the programmer writing the interface to the library, and the vector holding
+the values.
 
 What you should care about: API functions
 -----------------------------------------