Compute power metrics

[RobFryer]

Resolves #143

Computes the suite of power metrics required by AMAP. The user can tailor these using the new control argument to run_assessment; in particular by specifying the target power and the target detectable trend.

Have also made the construction of the original dtrend metric (detectable trend on the log-scale) compatible with the power control parameters.

Have tested (on a limited set of data) and updated the external data vignette to illustrate the use of the control argument with the target power and trend used by AMAP in it's last mercury assessment.

Have added some documentation and filled in some gaps in other documentation so that roxygen2 doesn't throw any warnings.

The power metrics are contained in a new element of each assessment list. They do not yet print out in the summary table. I will raise a new issue for this and deal with it when I work on the summary tables.

[hansmy]

Good to get more on the imposex and power ++ before release.