[Feature] Show warning if there are characters not in the FASTA specification

[Kronopt]

Is your feature request related to a problem? Right now warnings are only shown when calling complement (FastaSequence and LetterCode), at_gc_ratio or gc_content (FastaSequence) methods. No warning is shown when a sequence has characters not defined in the FASTA specification.

Describe the solution you'd like It might make sense to show a warning if a sequence has at least a single character not defined in the FASTA specification.