Added
* `cases` kwarg to `VarStat` to allow specifying which cases to use to calculate the stat ([GH-14](https://github.com/scottshambaugh/monaco/issues/14))
* `cases` kwarg to `dvars_sensitivity` to allow specifying which cases to use for the sensitivity analysis ([GH-14](https://github.com/scottshambaugh/monaco/issues/14))
* `cases` kwarg to `plot_2d_cov_ellipse`, `plot_integration_error`, and `plot_integration_convergence` to allow specifying which cases to use for the plot ([GH-14](https://github.com/scottshambaugh/monaco/issues/14))
Changed
* Switch varstat bootstrap method from 'basic' to 'BCa'
* `get_cases` function moved from `mc_plot` to `helper_functions`