resolved an issues raised by mypy

src/sim_explorer/case.py

@@ -871,13 +871,14 @@ def _init_new(self, case: Case, file: str | Path | None = ""):
         self.res = Json5(str(self._header_make()))  # instantiate the results object
         self._header_transform(tostring=False)
 
-    def _header_make(self):
+    def _header_make(self) -> dict[str, dict[str, Any]]:
         """Make a standard header for the results of 'case' as dict.
         This function is used as starting point when a new results file is created.
         """
         assert self.case is not None, "Case object not defined"
+        assert self.file is not None, "File name not defined"
         _ = self.case.cases.js.jspath("$.header.name", str, True)
-        results = {
+        results: dict[str, dict[str, Any]] = {
             "header": {
                 "case": self.case.name,
                 "dateTime": datetime.today().isoformat(),

src/sim_explorer/utils/paths.py

@@ -1,10 +1,9 @@
 from pathlib import Path
 
 
-def relative_path(p1: Path, p2: Path | None) -> str:
+def relative_path(p1: Path, p2: Path) -> str:
     """Identify the path of p1 relative to the file p2."""
     assert p1.exists()  # Note: the second path does not need to exist
-    assert isinstance(p2, Path)
     if p1.parent == p2.parent:
         return "./" + p1.name
     else: