Trailheads transform WIP

- This PR should:

  - Close #73. Each error now has a template. The only exception from the list in that issue is "steward_id". There should never be that error, since we create that file/value in the transformation process.
  - Close #43 by transforming fields indicating the presence of drinking water.
  - Close #45 by transforming fields indicating the presence of information kiosks.
  - Close #37 by transforming fields containing a unique ID for each trailhead feature.
  - Close #41 by transforming fields containing strings representing an address.
  - Close #42 by transforming fields indicating the presence of parking (including fields that indicate the presence of parking based on a few different strings, documented in the issue)
 
(Perhaps it is unconventional to indicate what a PR does *not* do, but it seems prudent to mention that...)

- This PR does not address:
  - #38 steward_id: Per the pattern of trail_segments, this code currently just assigns "0" as the steward_id for the trailhead. The steward_id created when stewards.csv is populated by the user via a form should replace this value at some point in the transformation process.

  - #39 trail_ids: it is not immediately clear to me how to grab the strings found in fields such as "trail name", then cross-reference those strings with the "name" field of named_trails.csv, grab the associated "id" for that row, and push each of those values into an array here.
  - #40 area_ids: we do not currently handle area files, and thus there are no area_ids to assign.


- Apologies for the duplicate commit message.