- Added functions 'cholesky_decomposition' and 'covariance_correlation_matrix'

- Added qualifier 'parallax' to the function 'cel2gal'
- Added qualifiers 'parallax_pma_corr', 'parallax_pmd_corr', and 'pma_pmd_corr'
  to the function 'orbit_calculator', which enables it to account for parameter
  correlations