Add logo source to examples, export logo w/ transparent bg & svg.

[tpapp]

• add the logo source to the manual, with calculations in Julia
• save the SVG and PNG (with transparent background) in the repository
• link the logo in the README from there

[tpapp]

I am investigating why this fails on Julia 1.0.5 only, but if in the meantime someone spots an obvious reason, please don't hesitate to share it.

[codecov-io]

Codecov Report

Merging #205 into master will increase coverage by 1.26%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #205      +/-   ##
==========================================
+ Coverage   85.37%   86.63%   +1.26%     
==========================================
  Files           9        9              
  Lines         581      591      +10     
==========================================
+ Hits          496      512      +16     
+ Misses         85       79       -6

Impacted Files	Coverage Δ
src/tikzdocument.jl	69.53% <0%> (-1.99%)	:arrow_down:
src/requires.jl	100% <0%> (ø)	:arrow_up:
src/axiselements.jl	95.34% <0%> (+0.58%)	:arrow_up:
src/axislike.jl	97.56% <0%> (+2.43%)	:arrow_up:
src/options.jl	90% <0%> (+3.33%)	:arrow_up:
src/build.jl	79.16% <0%> (+6.94%)	:arrow_up:

---

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[tpapp]

Everything is fixed now, the problem was that the docker image did not contain the relevant .sty file for LuaTeX.

[tpapp]

Alternatively, the same logo with colors from the Julia logo:

using PGFPlotsX, Colors

p1 = range(0, 360; length = 100)' # parameter 1
p2 = range(0, 360; length = 70)   # parameter 2
w1 = @. sind(3*(p1 + 3 * p2)) + 1.25 # wave 1
w2 = @. 6 + w1 * cosd(p1)            # wave 2
x = vec(@. w2 * cosd(p2))
y = vec(@. w2 * sind(p2))
z = vec(@. w1 * sind(p1))

@pgf Axis(
    {
        axis_equal,
        axis_lines = "none"
    },
    Plot3(
        {
            surf,
            z_buffer = "sort",
            colormap = "{Julia}{rgb255=(77,100,174), rgb255=(57,151,79), rgb255=(146,89,163), rgb255=(202,60,50)}",
            "mesh/rows" = length(p1)
        },
        Table(x, y, z)))

[tpapp]

A white stripe in the middle would ligthen it up somewhat:

[tpapp]

Changing the wave frequency makes it a bit simpler visually, which could work better for small size:

w1 = @. sind(3*(p1 + 2 * p2)) + 1.25 # wave 1

[KristofferC]

The bottom one looks really nice! Fine with me to swap to that one for the logo.

[tpapp]

Thanks for the feedback, I will merge this then tackle #188.