GCAM test data package

[rplzzz]

Right now it's hard to test packages that work on GCAM databases because the databases themselves are too large to conveniently include in the package. (This package is one of several that have this problem.) What we could do is create a separate package with a GCAM database in it, along with, possibly, a more comprehensive set of queries than the example queries in the rgcam package. Then we have packages that need a GCAM database just for testing Suggest the package with the sample database in it. Then we can write tests that use the database from the sample data package and have testthat skip those tests if the sample data package isn't available.)

The sample-data package will still be larger than recommended, but now the problem is confined to a single package that nobody has to install, which is a bit improvement over the current situation.