Fix issue #10

* Fixes issue #10 : `fst` module crashes when searching through the INFO field and finds unrelated "_AF" suffixes
* Module now searches for the pattern '_AF=' instead of '_AF'. This is
  far from perfect, but hopefully strikes a good balance between performance and stringency
* Partially fixes issue #8

Author: MaelLefeuvre

src/vcf-fst/src/lib.rs

@@ -378,7 +378,7 @@ impl<'a, 'panel> VCFIndexer<'a, 'panel> {
         let mut frequencies : BTreeMap<&str, f32> = BTreeMap::new();
 
         let pop_afs: Option<Vec<(&str, &str)>> = info.iter()
-            .filter(|&field| field.contains("_AF"))
+            .filter(|&field| field.contains("_AF="))
             .map(|field| field.split_once("_AF="))
             .collect();