pplacer should warn rather than die if an alignment looks like nt.

[matsen]

mus x/funfun » pplacer -c teeny.refpkg one.fasta
Running pplacer v1.1.alpha14-66-gfedfbc2 analysis on one.fasta...
Didn't find any reference sequences in given alignment file. Using supplied reference alignment.
Uncaught exception: Failure("You have given me what appears to be a nucleotide alignment, but have specified a model other than GTR. I only know GTR for nucleotides!")
Fatal error: exception Failure("You have given me what appears to be a nucleotide alignment, but have specified a model other than GTR. I only know GTR for nucleotides!")
Raised at file "pervasives.ml", line 22, characters 22-33
Re-raised at file "printexc.ml", line 71, characters 10-11
Called from file "list.ml", line 0, characters 0-0