bppsuite: refined typing

lib/bppsuite.mli

@@ -5,4 +5,4 @@ open File_formats
 val bppseqgen :
   nb_sites:int ->
   tree:nhx workflow ->
-  (* nucleotide_ *)fasta workflow
+  nucleotide_fasta workflow

lib/file_formats.ml

@@ -6,6 +6,7 @@ class type nhx = object
   method format : [`nhx]
 end
 
-(* type nucleotide_fasta = < fasta ; alphabet : [`Nucleotide] > *)
-
-
+class type nucleotide_fasta = object
+  inherit fasta
+  method alphabet : [`Nucleotide]
+end