Coding day 2015 electrostatic interaction

=Instructions=

1. Chose a nonbonded interaction to work on. 2. Find the relevant data structs used by this method and import them.
 * Expose this structs to Cython in src/python/espressomd/electrostatics.pxd

cdef extern from "p3m-common.hpp": ctypedef struct p3m_parameter_struct: double alpha_L double r_cut_iL int   mesh[3] ... 3. Find the c-function in the Espresso core which activates the interaction.
 * Most interactions have individual .cpp-files in src/core (p3m.cpp, p3m_gpu.cpp ...).
 * They typically contain a function such as

p3m_set_params 4. Expose this function to Cython in src/python/espressomd/electrostatics.pxd
 * Example:

cdef extern from "p3m.hpp": int p3m_set_params(double r_cut, int *mesh, int cao, double alpha, double accuracy) 5. In src/python/espressomd/electrostatics.pyx, implement a class derived from ElectrostaticInteraction.
 * Use the P3M class as example (main work).

6. Add a test case in testsuite/python/ElectrostaticInteractions.py.
 * These Tests only test the interface, i.e., whether parameters set equal parameters queried from Espresso