Use ndarray stats

[kylecarow]

This PR eliminates ndarrmax and ndarrmin and instead uses the methods implemented by ndarray_stats::QuantileExt

For example:

- let mc_max_eff = ndarrmax(&self.mc_eff_array);
+ let mc_max_eff = self.mc_eff_array.max().unwrap();

[kylecarow]

this might be useful for other features, see the docs here: https://docs.rs/ndarray-stats/latest/ndarray_stats/
but otherwise this can be reviewed & merged

[kylecarow]

@calbaker this should be easy to review, thought it would be nice using this crate's impl on ndarray::ArrayBase to provide min and max functions rather than our own versions

[kylecarow]

@calbaker @robinsteuteville related to our discussion today: it seems that Rust only compiles code in dependencies that is actually used!
https://www.reddit.com/r/rust/comments/7z5z6h/dead_code_elimination_in_rust/