write S3 `summary.maxcovr()` and `print.maxcovr()` methods for `max_coverage()`

[njtierney]

Summary would return key information such as:

• How many AEDs (Facilities) were selected
• How much coverage has improved by
• Average number of metres to nearest AED
• Information about the solver:
  • Solver type used
  • Time taken for solver to work

I would prefer that summary.maxcovr returned a dataframe.

Perhaps the print.maxcovr method would provide a "human readable" option.

[njtierney]

After some discussion with @noamross on the Slack, I'm going to go ahead with the following plan:

• print method is relatively concise
• summary is more verbose
• contribute to broom for tidy, augment, and glance.

tidy should be pretty straightforward, just a bit of a clean up on the existing methods in extract_mc_result...sorta. Currently it looks like:

n_added	distance_within	n_cov	pct_cov	n_not_cov	pct_not_cov	dist_avg	dist_sd	total_cost	install_cost	cost_relocate	n_proposed_chosen	n_existing_removed
0	100	339	0.1868798	1475	0.8131202	1400.192	1596.6756	NA	NA	NA	NA	NA
71	100	693	0.3820287	1121	0.6179713	560.345	719.4681	6e+05	5000	200	103	2

glance should just include

• n_cov
• pct_cov
• costs (if applicable - total, installation, relocation)
• time taken

augment - perhaps this could add columnar information about whether the original user is covered by a facility under the new model, and distance to the nearest new facility.

One thing that I need to do is record the length of time for the model to be fit.

[njtierney]

There are now print and summary methods for maxcovr.

Moving the broom::tidy focus into a separate issue.