Fix brittle merging of secchi data from multiple files

[jordansread]

This came up during work done in #191

Prior to that PR, WQP was the only source of secchi data. I linked LAGOS secchi and modified the munge_Kw() function to accept any number of secchi files. But to do that, we need to be able to expect that they have the same file format and column headers. Neither of these conditions were met by the two initial files: WQP is a feather with id instead of site_id, and lagos is an rds with site_id. I coded a temporary workaround because I wasn't sure what the downstream implications of changing the WQP file would be.

brittle code is here

• just like munge_H_A() we want order to matter in the file inputs
• need to lock in an expected file format and fail nicely (with a message making it clear why this failed) if the conditions are not met