Enable the limit_to argument from TikzPictures v3.2.0.

[mirkobunse]

The latest release of TikzPictures.jl offers the possibility to optionally omit the document and tikzpicture environments. The first of these environments could already be omitted in earlier versions of TikzPictures, by setting ìnclude_preamble=false. Omitting the tikzpicture is a new feature, which can be useful when embedding the generated TEX files in a larger document: JuliaTeX/TikzPictures.jl#56

This PR is bringing the new feature of TikzPictures to PGFPlots.jl. Backwards compatibility to earlier versions of TikzPictures is maintained.

Having the new TikzPictures v3.2.0

TEX files can omit the document environment by setting either include_preamble=false (like in earlier versions) or by equivalently setting limit_to=:picture. If the tikzpicture environment should also be omitted, you can set limit_to=:data.

Let us pin TikzPictures.jl to the new version and test the new feature:

using Pkg
Pkg.activate(".")
Pkg.pin(PackageSpec(name="TikzPictures", version=v"3.2.0"))

using PGFPlots # requires precompilation due to Pkg.pin
x = [1,2,3]
y = [2,4,1]
save("test.tex", plot(x, y); include_preamble=false) # works
save("test.tex", plot(x, y); limit_to=:picture) # equivalent result
save("test.tex", plot(x, y); limit_to=:data) # the new feature works, too

Having TikzPictures v3.1.0 (or older)

With earlier versions of TikzPictures, PGFPlots.save works just like before if no keyword argument is given, or if only include_preamble is specified.
This behaviour is achieved by only delegating the new limit_to argument if it is actually specified by the user.

Let us pin TikzPictures to an earlier version (don't forget to ] free TikzPictures afterwards).

using Pkg
Pkg.activate(".")
Pkg.pin(PackageSpec(name="TikzPictures", version=v"3.1.0"))

using PGFPlots # requires precompilation due to Pkg.pin
x = [1,2,3]
y = [2,4,1]
save("test.tex", plot(x, y); include_preamble=false) # works
save("test.tex", plot(x, y); limit_to=:picture) # has to break (feature not yet supported)

If PGFPlots.save breaks from a limit_to specification, it tells the user that the corresponding TikzPictures function does not exist. The message thus hints to the version problem and suggests to use include_preamble instead.

ERROR: MethodError: no method matching TikzPictures.TEX(::String; limit_to=:picture)
Closest candidates are:
  TikzPictures.TEX(::AbstractString; include_preamble) at ... got unsupported keyword argument "limit_to"

Due to this behaviour, we do not need to make TikzPictures >= v3.2.0 a hard requirement in the Project.toml. If a later version of TikzPictures is installed in the user environment (e.g. by ] up), the new feature will work. If it is not, it won't, but include_preamble is a working fallback solution.

[mirkobunse]

Thanks a lot for the quick merge!