refactor: update CLI

• Refactored cli.py, where the fasta dict couldn't be sliced
• Updated cli.py so the tool can now be installed with pip install . and works from the command line
• Refactored some of the code to fix CI pipeline failing
• Updated README

term_frag_sel/utils.py

@@ -23,5 +23,4 @@ def check_positive(value: str) -> int:
     except ValueError as exc:
         raise argparse.ArgumentTypeError(f"""Expected positive integer,
                                          got: {value}""") from exc
-    else:
-        return ivalue
+    return ivalue