Move original units/constants and add updated ones

[rieder]

AMUSE units are outdated: in some cases they contradict international agreed values. In this PR units and constants are updated to their current standards. As a reference, the default values used in Astropy should be used.

Changes include:

• yr is now the same as julianyr

• au value is updated

• solar/jupiter/earth related values updated

• [x] Make original AMUSE units / constants available under a new name

• [x] Update default AMUSE units / constants

• [ ] Update tests

• [ ] Test values of units against their Astropy equivalent

• [ ] Make sure that Astropy unit names map one-to-one to AMUSE units (create aliases where needed, maybe deprecate old AMUSE names)

[rieder]

Not all units/constants have been updated yet, but the most important ones have.