Feature: new InversionSolution having the mean rupture rate of the input solutions

[chrisbc]

I want to add the rates of two or more inversion solutions

adding a unique number to each rupture rate t e.g. calculating the mean rate of several ruptures https://github.com/GNS-Science/nzshm-runzi/blob/4929c3694a3d0212ff5f5a0c735876888cf53356/runzi/execute/average_solutions_task.py#L112.

rates_1 + rates_2

[chrisdicaprio]

@chrisdicaprio to clairify