Tiny bug fix

Merge pull request #4 from ChristopherMayes/master

Add small padding in charge deposition

Initial stable release

Completely new routines in open_spacecharge_mod.f90 to do the free space and cathode image charge calculation. The main routine is:

space_charge_3d(mesh3d, offset, at_cathode, calc_bfield), which performs the space charge calculation using the integrated Green function method and FFT-based convolutions. This can be used as a drop-in replacement for the old space_charge_freespace(csr%mesh3d), and is considerably faster.

Old code is still present and used by the test program in open_spacecharge_core_mod.f90

The example notebook runs the benchmark tests.