Modeler allocation input

[SouthEndMusic]

Fixes #656.

[SouthEndMusic]

For some reason I now get an SQLiteException("out of memory"), M'W'E:

using Ribasim
using SQLite

toml_path = "path/to/generated_testmodels/subnetwork/ribasim.toml"
cfg = Ribasim.Config(toml_path)
gpkg_path = Ribasim.input_path(cfg, cfg.database)
db = SQLite.DB(gpkg_path)
p = Ribasim.Parameters(db, cfg)
close(db)

Ribasim.generate_allocation_models!(p, db, cfg)

Edit: this is due to the database being closed before reading from it. The error message wasn't informative though, so @evetion fixed it: JuliaDatabases/SQLite.jl#337.

[SouthEndMusic]

@Reviewer: I suggest adding more tests after #659.

[visr]

Nice work! Left a few comments.