Support SIGNED debit and credit amounts in CSV convert command

[brucer42]

My credit union, Rivermark CU, uses negative amounts for debit in the debit field, positive for credit in the credit field.
Transaction Number,Date,Description,Memo,Amount Debit,Amount Credit,Balance,Check Number,Fees
"20180105000001[-8:PST]-70.55100**VZWRLSSIVR VN 800-922-0204 FL",01/05/2018,"VZWRLSSIVR VN 800-922-0204 FL","01-05-2018 DEBIT CARD DEBIT Eff. Date:",-70.55,,"256.06",,

I had some workaround where I appended an amount field from the debit and credit fields using ledger from before the 'convert command fix'. This worked because the amount I suppose overwrote the credit. That got fixed.

Unfortunately for me I updated ledger via brew and broke my scheme but I had become very dependent on the rich-data UUID's for splitting transactions into multiple accounts so I did not want to modify the 5 or so years of bank data. So I've added some further work-arounds to keep the UUIDs and use the latest ledger.

MY PROPOSAL: 1. put in a much clearer error msg in csv.cc such as "only one of credit, debit, or amount per xact".
2. enhance the convert command to allow specifying the sign of the field. Perhaps "...,-debit, credit, ..." in the 1st line. Very aesthetic, eh?"

On a side note, is there an expert someplace in building ledger on macOS?

Thanks a ton for this software