various small changes

- generalized 'trackcpp_create_rms_folders.m' (now if no argument is
  passed opens UI dialog window for user to select input file)
- small modifications and implementations in fieldmaptrack, dipole
  analysis and track. Added a Trajectory method that finds the
  interpolation point on the trajectory given a SerretFrenet
  coordinate system.
- changes path of sirius_lattice folder. Now this folder and its
  corresponding modules with lattice definitions are under 'sirius/si'
  or 'sirius/bo'
- added 'sirius/system.py' to put system-dependent initialization
  structures.