Small update to deal with IUPAC ambiguity codes.

Modelling of base calling errors within Art.py does not currently support ambiguous symbols besides N.

This release adds the following support:

• informative error reporting when reference sequences contains symbols other than ACGTN.
• new command line option --convert filters references in memory, converting unsupported symbols to N.
• --simple-reads option does not model errors and can handle all symbols.