Split package into several modules

[lnacquaroli]

It could be a good idea to split the packages somewhat back to what they were before. I haven't analyzed this in-depth but could help in the maintenance and development as well. Not sure how much will impact current users though. Maybe we could split the package while keeping this one active.

ThinFilmsTools.jl:

• OpticsMatrixMethod.jl (2x2 matrices, linear optics)
• ThreeOmegaMethod.jl
• UtilityTools.jl: the regular tools used should be imported maybe from another independent package.
• RefractiveIndexDB.jl
• RefractiveIndexModels.jl