Atom selection dummy update

[ChiCheng45]

Description of work
Changed the all selector so that it does not include dummy atoms. Added a selector for dummy atoms. The default is now changed so that all atoms except dummy atoms are selected.

Fixes
Closes #352.

To test

1. Load a trajectory with dummy atoms.
2. Using the atom selection run a analysis with and without dummy atoms.
3. Load up the MDA file and check that dummy are or are not included.
4. Load a trajectory without dummy atoms.
5. The option to select dummy atoms in the atom selection helper should be greyed out.

[MBartkowiakSTFC]

I tried this version out on trajectories with and without dummy atoms, and ran an analysis for each case (dummy atoms included, dummy atoms excluded). I did not run into any problems.