Change 'basin' flag to 'usgsBasin' or similar

Currently, most active-crust and subduction GMMs use basin as the flag to indicate whether to apply the USGS deep (depth-tapered) and long-period basin (de)amplification. This is easily misinterpreted to mean basin effects (terms), generally, are turned on or off.

mentioned in issue #254 (closed)

added Enhancement GMM labels

We'll leave the Gmm.ID_BASIN along but will improve documentation of what this means.

mentioned in merge request !431 (merged)

mentioned in commit 9dbdcf3b

closed with merge request !431 (merged)