rename getKey to getString

pdb-api/src/main/java/org/lucares/pdb/api/StringCompressor.java

@@ -49,7 +49,7 @@ public class StringCompressor {
 
     public String getString(final int integer) {
 
-        return usip.getKey(integer);
+        return usip.getString(integer);
     }
 
     public int getIfPresent(final String string) {

pdb-api/src/main/java/org/lucares/pdb/api/UniqueStringIntegerPairs.java

@@ -184,7 +184,7 @@ public class UniqueStringIntegerPairs {
         return stringToInt.get(string);
     }
 
-    public String getKey(final int second) {
+    public String getString(final int second) {
         return intToString.get(second);
     }