Add labelmap2vec.

[holtri]

This adds functionality to convert a labelmap into a label vector.

Example:

julia> labelvec = [:yes,:no,:no,:yes,:yes]
5-element Array{Symbol,1}:
 :yes
 :no
 :no
 :yes
 :yes

julia> lm = labelmap(labelvec)
Dict{Symbol,Array{Int64,1}} with 2 entries:
  :yes => [1, 4, 5]
  :no  => [2, 3]

julia> labelmap2vec(lm)
5-element Array{Symbol,1}:
 :yes
 :no
 :no
 :yes
 :yes

[coveralls]

Coverage decreased (-14.4%) to 85.593% when pulling 322beb2 on holtri:lm2vec into a21c5b5 on JuliaML:master.

[coveralls]

Coverage decreased (-14.4%) to 85.593% when pulling 51d06b5 on holtri:lm2vec into a21c5b5 on JuliaML:master.

[holtri]

Coverage decrease seems to be related to issues with coverage reporting: JuliaCI/Coverage.jl#187
JuliaLang/julia#28192

[Evizero]

I can see how this could be useful, thanks!

I order for me to merge could you do one of the following two things:

• add a docstring to labelmap2vec and an entry for it in the docs.
• not export labelmap2vec

[oxinabox]

I think the docstring should mention it is the inverse function of labelmap

[Evizero]

thanks!