DataSet configuration

[c42f]

This change allows dataset configuration to be modified via the DataSets.config!() API.

A notable internal change is that this requires each DataSet to be owned by one project, such that dataset changes can be written back to the project Data.toml (or other dataset project storage, such as JuliaHub).

Now that datasets can be mutated via the API, I found it necessary to remove and deprecate the crude @__DIR__ templating mechanism for local file storage dataset paths, and simply specify that relative paths are relative to the location of the Data.toml, via the local_data_abspath() mechanism.

---

@jeremiedb, here's the user-facing documentation:

    config!(name::AbstractString; kws...)
    config!(proj::AbstractDataProject, name::AbstractString; kws...)

    config!(dataset::DataSet; kws...)

Update the configuration of dataset with the given keyword arguments and
persist it in the dataset's project storage. The versions which take a name
use that name to search within the given data project.

Examples

Update the description of the dataset named "SomeData" in the global project:

DataSets.config!("SomeData"; description="This is a description")

Alternatively, setting DataSet properties can be used to update metadata. For
example, to tag the dataset "SomeData" with tags "A" and "B".

ds = dataset("SomeData")
ds.tags = ["A", "B"]