Change order of arguments for adding auxiliary data for 3.0 release

[BFedder]

Is your feature request related to a problem?

Currently, auxiliary data is added as such:

aux = mda.auxiliary.EDR.EDRReader(edr_file)
u.trajectory.add_auxiliary(aux_spec, aux)

i.e.

u.trajectory.add_auxiliary({"temp": "Temperature"}, aux)

Adding all data is done as such:

u.trajectory.add_auxiliary(auxdata=aux)

aux_spec and aux_data both are optional arguments for this solution. Changing the order and making auxdata mandatory is preferable, but is a breaking change to the aux framework.

Describe the solution you'd like

In 3.0, the order of aux_spec and aux_data should be switched, and only aux_spec should be optional and default to None. Then, adding all auxdata could be done as such:

u.trajectory.add_auxiliary(aux)

This will require changes in coordinates.base.add_auxiliary and the tests. I have left a TODO comment there.

Describe alternatives you've considered

Leave as is

Additional context

For more context on how the auxiliary system works, see my blog

[orbeckst]

You could create the PR and it will wait until we are getting ready for 3.0. According to @IAlibay on twitter, this will be September 2023. So no rush... but it would be nice to have it queued up, especially if it only touches code related to aux.

[orbeckst]

We also need a reminder to deprecate the old functionality — that should go into 2 releases prior to 3.0.