Refactor next_to and align_to

[leotrs]

There are several issues with mobject.next_to and mobject.align_to.

1. align_to accepts an argument called alignment_vector that is never used in the code.
2. align_to has a docstring that is stale (it refers to this alignment_vector argument that is never used).
3. Both next_to and align_to accept a mobject or a point as target, but next_to accepts the specification of a submobject, while align_to doesn't. For symmetry and completeness, I think align_to should also accept a submobject.

[FominSergiy]

Hey guys! I am looking into this issue and hopefully can contribute. Just a quick question around the "accepts the specification of a submobject" mentioned above.

the specifications are two parameters that are present on next_to method, specifically:

submobject_to_align=None,
index_of_submobject_to_align=None

and essentially the update would be to add these two params to the align_to method and update the isinstance block statement on align_to to what the next_to method has.

let me know if this makes sense :)

[FominSergiy]

Or is it all these params as well?

buff=DEFAULT_MOBJECT_TO_MOBJECT_BUFFER,
aligned_edge=ORIGIN,
submobject_to_align=None,
index_of_submobject_to_align=None,
coor_mask=np.array([1, 1, 1]),

[Darylgolden]

Removed good first issue label, I don't think it is one.

[ad-chaos]

Both next_to and align_to accept a mobject or a point as target, but next_to accepts the specification of a submobject, while align_to doesn't. For symmetry and completeness, I think align_to should also accept a submobject.

By doing this wouldn't next_to and align_to become identical methods? Hmm, at that point we could probably nuke one of them. Thoughts?