Deprecate emu unit

[pllim]

Fix #4918 , where user was confused by the "emu" unit due to ambiguous definition of that term.

Supersede #5787 .

A subset of #5793. And this follows one of the suggestions there by @mhvk . Perhaps @eteq can utilize this new module for his work on #5661.

[mhvk]

@pllim - I like this approach! I wonder, though, whether it wouldn't make sense to follow imperial a bit more closely and just define the units and use add_enabled_units in enable. In the comments you note possible name clashes. Are you thinking of something specific? The prefixed earth/jupiter mass? I think this might well "just work" even with add_enabled_units... (but have not checked).

Also, obviously needs some mention in the documentation...

[pllim]

@mhvk

Re: "add" vs "set". No particular reason. It was just cut-and-paste. And apparently I chose the wrong module. Fixed that in the second commit.

Re: Documentation. It is auto-generated. The following will appear at the end of the API documentation. Given that the unit is deprecated, I decided against giving it extra publicity, as we do not want to encourage people to use it (but it is there if they really, really want to).

[mhvk]

This now all looks good!

[mhvk]

@pllim - OK, all looks good. Feel free to merge once the tests have passed.

[MSeifert04]

hm, deprecated.enable() sounds like a very good 2017-4-1 feature. 😄

[pllim]

We can always wait till Apr 1 to merge, it is not that far away. 😂

[bsipocz]

@pllim @mhvk - I agree that this doesn't need much publicity, thus I wonder whether we even need the table of the deprecated units, or the description above the table should be enough?

[pllim]

@bsipocz , then there is no easy way to find out what unit is deprecated. Also, this provides a consistent API presentation. So, I am against removing the table but I will if overruled.

[pllim]

Since this is approved and tests passed, merging.