Keep dependency in memory if loading into PostGIS

[kzollove]

Currently, if a dependency for a variable (attr or geom) is not in the database, the importShapefile will go through a download-upload-import cycle.

The download is from the web-hosted data source into the R environment, where it is transformed into the standard format.

The standardized data is then uploaded to PostGIS.

Now that the dependency exists in PostGIS, the attr and geom are joined and together they are imported back into the R environment.

We should be able to skip the import step if we already have the dependency in memory. The snag here is attr and geom are joined in PG and then imported. There are three cases to address:

1. neither dependency existed in the DB and was just downloaded into the R environment
2. geom didn't exist, was just downloaded, and attr needs to be imported from DB
3. attr didn't exist, was just downloaded, and geom needs to be imported from DB

This is an important issue and could help ease the memory strains and help with #8

[kzollove]

Could be handled with boolean arguments in loadVariable and loadGeomTable: keepInEnvironment=TRUE

[rtmill]

Seems like this is more of a general problem of - indicating/checking for that a given data source is already loaded?

[kzollove]

The inefficiencies related to redownloading were addressed in a52acd0ea173d94153419a505bff9ccd354891c3, but that doesn't fix this issue.