GeoPackage validation routine

[brownag]

• check for required tables
• check for consistency between tables (foreign key relationships)
• check for duplicate entries within tables (violated primary key constraints)
• ? https://github.com/brownag/gpkg/blob/c813561bb6435df376fef39a20108cef956eb4a0/R/gpkg-validate.R#L1-L12

[brownag]

• In 37029a18a49d3e36df42d6c5e85fb537c7fa90fb implemented basic gpkg_validate() routine (will be expanded)
  • Checks that gpkg_contents and gpkg_spatial_ref_sys exist
  • Checks that at least one tile or vector dataset (with at least 0 rows) is in the database and gpkg_contents

I think it should not be necessary to check for duplicate records--as unique constraints that (should be) set when creating required tables will cover this. If functions outside this package, and non-standard SQL is used, then the file is not a GeoPackage--technically speaking.

I don't think it is worth necessarily validating all triggers/unique/key constraints, but perhaps checking required gpkg_contents and gpkg_spatial_ref_sys table definitions against a lookup table would be a good addition. GDAL does it properly, the methods in this package do it properly, so it would simply be a protection against other avenues that might create these tables, or the user running custom gpkg_execute() statements