Adding nix shell and reworking some parser/local file manager stuff

SzLib/ISzFileManager.cs

@@ -1,8 +1,12 @@
+using SzLib.DataObjects;
+
 namespace SzLib;
 
 public interface ISzFileManager 
-{
+{ 
     public string DataPath {get;}
-
+    public string DatasetsPath {get;}
+    
     public bool SaveFile(string path, string fileContent);
-}
+    public string? LoadFile(string path);
+}