dynamic GeoPackage file writing

[brownag]

Creation of a geopackage object should collect or create all the required information to take any associated R data objects and user input and create a GeoPackage file. There is no mechanism to do the whole process currently, the user has to build a new file incrementally with gpkg_write(<object>, ...), possibly appending to an existing file.

Currently you can also create geopackage S3 objects that are not "backed up by" (or derived from) any file. This is not currently a useful thing to do, the object is really only "read only" by the time the R user sees it. No changes they make to the R object can be done to the geopackage without going through the SQLiteConnection.

I would like to have a gpkg_write(<geopackage>) method that will essentially figure out the sequence of layer writes, table updates etc. that are necessary to create a (possibly complex) GeoPackage directly from an R object (i.e. a list of layers)

• This could potentially be automatically invoked, creating a geopackage that exists only as a temporary file
  • The user should not necessarily have to be concerned with how the file is put together or where it is just that they had some R objects that they wanted to be able to get from it.
• It also should be possible (at least in theory) to "roundtrip" arbitrary geopackages in to the S3 object representation and back out as a new geopackage file with identical contents. I am sure there are some possible wrinkles here, but should be achievable for simple cases.

[brownag]

This has all been folded into the geopackage() constructor in v0.0.5.

• If you do not specify dsn then a temporary geopackage file will be created at time of construction.
• If dsn is specified (but does not exist), it will be created when x is a (optionally named) list of objects to add to the database.
• If dsn exists it is not updated. Attempts to update an existing dsn via the geopackage() constructor will give an informative error. gpkg_read(), or the non-list constructors, can be used to load an existing database as a geopackage object from file and gpkg_write() is used to append one (or more) layers. gpkg_write() is necessary to gain full control over all possible layer creation options.