Add HelloWorld plugin

To get to know the structure of plugins.
Add TouchParticles plugin

This is a little plugin desgined to touch each particle in their respective
cell and supercell. It intends to just write out attributes of the particles
at each execution.
Make ionization folder

Change of plans. There will be no ionization plugin anymore. It is crucial
when ionization happens, so it will be put directly into the PIC cycle.
This commit contains uncommited changes to old non-working plugins to
touch the attributes of particles and a new folder called "ionization"
with an "ionization.hpp".
Orthography

Corrected some orthography - in the comments of course, not in the
functional code. :P
Last changes for ionization plugins - then abort

These most recent changes to the non-functional plugins for "touching the
particles" will only be committed to be used for writing a guide as to how
plugins are witten. Then they will be removed because ionization will not
be implemented in the form of plugins anymore but directly into the code.
Delete plugin approach, modify Particles.*

The new approach is to implement ionization directly into the code.
Therefore the old plugins have been deleted and files like Particles.*
and respective configuration files have been modified. This was done in
analogy to how pushers are implemented. The progress so far:
 - there is an empty ionization framesolver in the right structure
 - it produces output and says "Hello!"
Remove GetAttributes plugin

Remove the rest of the plugins that were useless but which I forgot before.
Introduce chargeState particle attribute

The charge state is given in integer numbers and will then be
multiplied with the weighting to get the charge of the macro particle