Snaphot 2.0

[Kraysent]

Would like to refactor snapshots. For now they are as simple as AMUSE Particles object + timestamp atached to it. This makes requirements for installation more complex and code itself as well.

Some of things to change are:

• Get rid of native AMUSE Particles object and use pandas DataFrames with fields to store each snapshot. This would make AMUSE requirement generally not mandatory (with some refactoring along the way).
• Use astropy's native units in all of the general computations. They support ndarrays which makes them suitable for vectorised computations and since astropy has much easier reqirements it would be easier to install.
• Add general functions to Snapshots like computation of center of mass and so on.