The brentq method gives better results at very small T/T_F, where the fugacity becomes
exponentially large.

- Add fermi radius and h.o. length to fermi gas calculated quantities.
- Change printout of total entropy and energy in fermi gas to per particle value.
- Add rotated waist computation to laserbeam class.

- providing a transition object is no longer needed
- definition of the fermi gas is done exclusively via trap frequencies
- add docstring for fermi gas
- add option to either pass absolute or reduced temperature
- add option to turn off parameter printout via verbose argument

- Trap parameters (frequencies, depth, potential) accept polarizability instead of a Transition object, allowing to use
   calculated/measured polarizabilites.
- Transition object has a function get_polarizability, which can be used if a single-transition approximation is sufficient.

- added gravitational constant
- added function to directly return optical dipole potential in single beam dipole trap.

- add single beam dipole trap
- fix calculation of peak power (factor 2 was missing)
- add possibility to use elliptic beams for accordion lattice.

Fix polylog without fdint

See merge request !3

Changed the way the traps are defined: Now via individual 'LaserBeam' objects, to allow for more flexible trap geometries. Added a preliminar implementation of the loading model.