Restucture project files

[akabla]

• definition.jl is getting large. I suggest to split it into rheodata.jl (including log management functions) and rheomodel.jl

• Separate processing.jl into dataproc.jl for rheodata filtering, and modelproc.jl for fitting and predicting based on moduli.

• Have file for NLopt specific objective functions and fitting methods