sql.py: put_table(): Wrap repr() calls for debug messages in strings.as_tt() to add Redmine formatting

sql.py: put_table(): Output "Adding index" debug message with level=2.5 so it's not part of the Redmine steps

schemas/vegbien.sql, functions.sql: Cast functions: Fixed bug where invalid value exceptions were not being caught, because implicit conversions to the return type apparently only happen outside the block containing the RETURN statement (i.e. at the end of the function). Fixed by adding explicit type conversion to return type, so that type conversion would happen inside try block.

sql.py: put_table(): Re-enabled FunctionValueException handling, by just filtering out the value on all input columns that use the named function (since the error message does not specify which column it was that had the invalid value). This is in some ways better, anyway, because that way the invalid value is filtered out right away in all columns that could contain it, instead of potentially once for each column (if the value appears in more than one input column).

sql.py: add_index(): Fixed bug where expressions could not be converted to a string until their table name had been removed

sql_gen.py: Added Expr

sql.py: add_index(): Fixed bug where expressions needed to be enclosed in () to distinguish them from plain columns

sql.py: add_index(): Support simple expressions as well as columns

sql.py: Renamed index_col() to add_index() so its name isn't similar to index_cols()

sql_gen.py: FunctionCall: Removed repr() because it's a Code object and its to_str() does not take extra arguments