BUG: Errors in HWSD2 dataset

From @minxu74:
-  Error in kg/m2 conversion (need to multiple bulk density by 1000 and divide organic carbon by 100)
- Soil thickness should be calculated via `bottom_depth - top_depth` but was just `bottom_depth`
- Did not actually use the "stock" variable that was created
- After fixing the above, global attributes should be updated
- Suggest including soil level translations; i.e., D1 = 0-20cm