Still failing the internalGenerator testcase - will need another look at this

@HomesGH Tests passing and ready for review.

To be honest, I still don't see the benefit of changing it to the invalid Q(0,0,0,0). Is it just performance-wise why we don't choose the identity Q(1,0,0,0)? There is much worse performing code, eg. the check for duplicated IDs.

For me, it would be ok to initialize the Quaternion class itself with 0,0,0,0, but all molecules should always carry valid data (Q(0,0,0,0) is invalid). Therefore, I see two options:

1. Use the identity quaternion for initialization
2. Use no defaults for the molecule class. All parameters have to be set by other classes/methods/etc. This might not be in line with the paradigm "Convention over configuration". It might also cause further trouble, see my comment below.

Furthermore, I can not guarantee that there are no hidden molecule initializations somewhere in the code. Those could break when using invalid data for initialization.

These lines could also cause trouble, even though, they should not be relevant in theory.

Also the PerCellGenerator misses the explicit passing of the quaternions.
Same here:
ParticleCellBase.cpp:154
DensityControl.cpp:374
PerCellGenerator.cpp:123

I can not guarantee that I found all Molecule/FullMolecule inits that use some of the defaults.

I address the concern about missing initialization values by changing the Molecule constructor to require all parameters to be passed - removing default values from the interface. This should catch all places during compile time!

I address the concern about missing initialization values by changing the Molecule constructor to require all parameters to be passed - removing default values from the interface. This should catch all places during compile time!

This is ok-ish for me. Even though, always having to set all of those zeros including a single 1 looks a bit error-prone to me. E.g. accidentally setting vz to 1 instead of qw is simple:
Molecule m(id, comp, rx, ry, rz, 0., 0., 0., 1., 0., 0., 0., 0., 0., 0.);
versus
Molecule m(id, comp, rx, ry, rz, 0., 0., 1., 0., 0., 0., 0., 0., 0., 0.);