Represents "custom" fields (ie non-predetermined fields) that can show up in
the list of orders
These can appear only as integers, and their value are treated as either
ints, doubles or strings (catch-all)
Ex: Price,OrderQty,1324,Account
For some fields (day, side, etc) we do custom lookups since
the orders may be "DAY", MKT (for price), etc
For all others, delegate to the basic field type lookup