Refactor: Reorganise dft_lib files according to their namespace

[migduroli]

The Lib folder is full of useful classes which could be reorganised in the following folder structure. This could be useful for new contributors to easily identify the module they can start contributing to (in a more controlled way):

• [ ] Physics
  • [ ] Potentials
    • [x] InterParticle [e.g. tWF, LJ, VdW] (e8fecbdb3)
    • [ ] External [LJ(9, 3)]
  • [ ] FreeEnergy:
    • [ ] Bulk [e.g. Enskog]
      • [ ] Enskog
    • [ ] Density Functional Theory [GSA, FMT, ...] (I don't know whether you're thinking of including other models)
      • [ ] DFT
      • [ ] FMT
  • [ ] Fields:
    • [ ] Density (I think the density class would be just a materialisation of a Field template class)
• [ ] Numerics
  • [x] Arithmetic (3c332a60)
  • [ ] LinAlg
    • [ ] Poly
    • [ ] Poly34
  • [x] Integration (cac40135c)
  • [ ] Optimization
    • [ ] Minimizer
    • [ ] FIRE2 minimizer
    • [ ] String method
    • [ ] String method MPI
  • [ ] Solvers
    • [ ] Dimer method
• [x] Geometry (e6754a20):
  • [x] Point (e6754a20)
  • [ ] (Edge?)
  • [x] Element (e6754a20)
  • [x] Lattice/Mesh (e6754a20)
• [x] Graph
  • [x] Grace (cdaca20f)
  • [ ] (Gnuplot?)
• [ ] Utils:
  • [ ] IO / VisitWriter
  • [ ] IO / ReadTable
    • [ ] Table
  • [ ] Log / TimeStamp
    • [ ] TimeStamp -> console::_now() (this could be made publicly available)
    • [ ] Log
  • [x] ConfigParser (options) (e8402e0a)

[jimlutsko]

This sound like a good idea but one small thing: I think the Util VisitWriter should be moved to be a method on the Density object as its only purpose is to write the density in a form that can be used by vtk-based graphics programs.

[migduroli]

I've tried to update how I see the final dir-tree will be, including reference to the classes/files affected (from legacy_lib). If you see anything wrong or missing, please do not hesitate to modify it.