Implement BB full with gamma

[davidwalter2]

For completeness, the BB full with gamma distributed beta parameters is implemented. This requires explicit parameters.

• One parameter per bin is introduced defined as $n_\beta = \sum_j n_j \beta_j / \sum_j n_j $, to decouple the system of equations, the solution for each $\beta_j$ parameter is then computed analytically.
• The $n_\beta$ are appended to the end of the fitter.x tensor
• Impacts and global impacts are implemented for this mode by stripping off the additional parameters
• The EDM for beta parameters in the full scheme is fixed by masking bins/processes that don't contribute

[cippy]

Isn't this PR implementing it ?

[davidwalter2]

This is w.r.t. the toys, I'm not sure but I think one would need to compute and vary the $n_\beta$ parameters here.