adding likelihood code for the anisotropic CMASS BAO measurement from th...

...e BOSS collaboration

changes in detail:
- added gridded likelihood file (in a alpha_perp, alpha_para grid) for this measurment
- added identifiers to data/bao_2013.txt
- added a new Likelihood class `bao_boss_aniso` for this measurment using the likelihood grid
- added a new Likelihood class `bao_boss_aniso_gauss_approx` which approximates the true likelihood by an Gaussian prior
  (NOTE: in contrast to `bao_boss` the correlation between H and Da measurements is taken into account)
- minor whitespace adjustment in bao_known_rs.data
- the `bao_boss` class is changed such that any anisotropic BAO measurement using the CMASS sample prohibits the isotropic CMASS measurment to be taken into account
- including `bao_boss_aniso` and `bao_boss_aniso_gauss_approx` at the same time is prohibited