Support Kreuzer-Skarke list for polytopes of dimension 3

[HereAround]

cc @ariostas

(Maybe we could even go beyond dimension 3,4,5?)

[ariostas]

For dimensions 1,2,3 we can simply put them in a file and ship it with the package. For dimension 5, no one has the full list because it is ridiculously big. Schöller-Skarke have a list of the "generating" polytopes from where one can in principle get the full list. These polytopes can be obtained with the fetch_polytopes function, except that PALP is needed to convert them from a weight system representation into a normal vertex representation. If you don't want to add PALP as a dependency we can also write code to do this last step.

[HereAround]

I was mostly thinking about dimensions 1,2,3. Either reading it with an (extended) fetch_polytope method, or as you suggest by adding them as files.

Regarding dimension 5 - I was under the impression that your function can fetch 5-dimensional polytopes. Am I mistaken? For right now, PALP is not included (yet). (In fact, this is the part of your code, that is not yet tested according to codecov and which I wanted to increase with https://github.com/HereAround/CYTools.jl/issues/22).