aerocargo/csv - add option for stock adjustment

Can we please look at adding an option into the aerocargo/csv module to allow us to adjust existing stock levels instead of overriding them? Options from Tim: 1. A new column available in the DataPort (underlying Aero) CSV importer for a column Stock Mutation, which can either be Increment or Decrement. This could then be paired with the Stock Level column. 2. Two new columns, Stock Level Increment and Stock Level Decrement 3. A new column Stock Level Mutation which would be positive numbers for adding and negative for subtracting stock 4. Something similar to the above but with different words, like Adjustment instead of Mutation