StepSnapH5 only built if timeh5 exists

StepSnapH5 also now directly holds the path to timeh5 instead of
the entire `sdat` instance.

Author: amorison

src/stagpy/_caching.py

@@ -12,6 +12,7 @@
 
 if typing.TYPE_CHECKING:
     from collections.abc import Mapping
+    from pathlib import Path
 
     from .datatypes import Field
     from .stagyydata import StagyyData
@@ -89,15 +90,14 @@ class StepSnapInfo:
 
 @dataclass(frozen=True)
 class StepSnapH5(StepSnap):
-    sdat: StagyyData
+    timeh5: Path
 
     @cached_property
     def _info(self) -> StepSnapInfo:
         isnap = -1
         step_to_snap = {}
         snap_to_step = {}
-        timeh5 = self.sdat.par.h5_output("time_botT.h5")
-        for isnap, istep in parsers.h5.extras.isnap_istep(timeh5):
+        for isnap, istep in parsers.h5.extras.isnap_istep(self.timeh5):
             step_to_snap[istep] = isnap
             snap_to_step[isnap] = istep
         return StepSnapInfo(

src/stagpy/stagyydata.py

@@ -799,6 +799,7 @@ def _field_cache(self) -> FieldCache:
 
     @cached_property
     def _step_snap(self) -> StepSnap:
-        if self.hdf5 is not None:
-            return StepSnapH5(sdat=self)
+        timeh5 = self.par.h5_output("time_botT.h5")
+        if timeh5.is_file():
+            return StepSnapH5(timeh5=timeh5)
         return StepSnapLegacy(sdat=self)