Added sagittal / tangential support for tilted elements

[remi-gj]

added :

• GaussianBeam3d for independant sagital & tangential planes
• "Elements" type, which include both Element and Element3d
• Element3d type (ThinLens3d, ThickLens3d, Interface3d, Mirror3d) for theta-tilted elements ( = like a misalingment). for each of the elements needing a radius of curvature, R>0 for a concave surface ie the center is before the interface ")" (this was the case for the mirror, but not the interface, maybe it should be mentioned in the doc)
• the propagation with Elements (if at leat 1 Element3d, will result in a GaussianBeam3d)
• gaussian3dplot : did not succeed returning 2 shapes for plotting using the @ receipe and @ series so i made a custom function (might be changed)

if i made any mistake or if anything is not clear or need to be changed, feel free to ask !

[roflmaostc]

Hi,
huge effort!
It already helps to add some tests and see if everything still runs (especially old tests).

I'll try to give a more detailed review

[roflmaostc]

One question: Do we need Lens3dor can't we just use the ThinLens as before but with generic options?

[remi-gj]

do you mean by using a single ThinLens returning either a ThinLens or a ThinLens3d (this would be possible, i can try to do it), or keeping only the Element type, expecting the user to then ask for a sagital or tangential view of the system (like in https://github.com/ngedwin98/ABCDBeamTrace.jl/tree/master but i think it's less intuitive)