create tmp folder if does not exist

[dringeis]

I've made two tiny changes to the Makefile so it works for me on macOS just after cloning :

• remove a / in front of the %.pdf names, it was looking for the tex file on root.
• make sure that the tmp folder is created if it does not exist, otherwise commands fail.
  • Maybe the tmp folder could be deleted on make clean, not sure if necessary

To be tested if a GNU/Linux platform before merging, maybe behaviors are different.

[dringeis]

Added a solution for the differences in color between the arrow and the gradient, following this discussion: https://tex.stackexchange.com/questions/684932/i-use-color-with-left-color-right-color-and-fill-in-tikz-but-why-are-these-two