rename delta->gamma

[peterstangl]

The "tree" CKM parameterizations that is used by wilson uses the Unitarity Triangle (UT) angle $\gamma$ to determine the CKM phase $\delta$. Since $\delta=\gamma$ is a very accurate approximation, so far $\delta$ was set to $\gamma$ everywhere in wilson and the parameter corresponding to the angle $\gamma$ has been called delta. This is changed by the present PR, which now distinguishes $\gamma$ and $\delta$ and uses the name gamma for the UT angle $\gamma$.
This change is motivated by the implementation of the exact relation between $\gamma$ and $\delta$ in the ckmutil package in PR flav-io/ckmutil#4.

Codes that update wilson's default CKM parameters currently update the value of the parameter delta and not gamma. To preserve backward compatibility, this PR introduces a temporary fix that sets the parameter gamma to the value of the parameter delta in case the parameters option is set on the Wilson class.

[DavidMStraub]

If it's temporary, I suggest issueing a warning that this is deprecated.