This PR

• updates the previous implementation of CKM parameterizations, replacing approximate by exact relations,
• adds the new $\beta$ - $\gamma$ parameterization, in which all CKM elements are expressed in terms of the CKM elements $V_{us}$ and $V_{cb}$ and the Unitarity Triangle angles $\beta$ and $\gamma$.

The changes to the previous implementation are:

• The new function gamma_to_delta implements the exact relation between $\delta$ and $\gamma$ but its optional argument delta_expansion_order allows to use the very accurate approximation $\delta=\gamma$ for delta_expansion_order=0 or to include higher-order corrections to this approximation for delta_expansion_order=1 or delta_expansion_order=2.
• The new functions tree_to_standard and wolfenstein_to_standard, and their inverses standard_to_tree and standard_to_wolfenstein are introduced. They all use exact relations without analytical approximations. In particular, the exact relation between $\bar\rho + i \bar\eta$ and $\rho + i \eta$ has been implemented for the Wolfenstein parameterization and the exact relation between $\delta$ and $\gamma$ is used everywhere (but the new optional argument delta_expansion_order allows to use approximate relations).
• The old function tree_to_wolfenstein is now a composition of tree_to_standard and standard_to_wolfenstein and its inverse wolfenstein_to_tree is added as a composition of wolfenstein_to_standard and standard_to_tree.
• The old function ckm_wolfenstein is now a composition of wolfenstein_to_standard and ckm_standard.
• The old function ckm_tree is now a composition of tree_to_standard and ckm_standard.

The newly implemented $\beta$ - $\gamma$ parameterization consists of:

• The function beta_gamma_to_delta that implements the exact relation between $\delta$ and $\gamma$ if $\beta$ is known and its optional argument delta_expansion_order allows to use the very accurate approximation $\delta=\gamma$ for delta_expansion_order=0 or to include higher-order corrections to this approximation for delta_expansion_order=1 or delta_expansion_order=2.
• The new function beta_gamma_to_standard and its inverse standard_to_beta_gamma implement the translations between the $\beta$ - $\gamma$ parameterization and the standard parameterization.
• The new function ckm_beta_gamma implements the CKM matrix in the $\beta$ - $\gamma$ parameterization in terms of a composition of beta_gamma_to_standard and ckm_standard.

The unittests have been updated and extended.