Sanitize cal_tools.tools.get_creation_date

This is just a little bit of maintenance, as I'm wading through the code.

This MR cleans

It was tested so:

In [1]: from cal_tools.tools import get_dir_creation_date

In [2]: folder = "/gpfs/exfel/exp/DETLAB/202031/p900172/raw"

In [3]: get_dir_creation_date(folder, 10)
Out[3]: datetime.datetime(2020, 7, 20, 12, 39, 20, 631633)

In [4]: get_dir_creation_date(folder, 4)  # no such run, None is returned

In [5]: 

One question maybe is, should this function return None when the date could not be extracted, as it is now, or to raise?
I'd prefer to raise an error, so that it's clear that this step failed, rather than failing later.

@kamile @ahmedk

cal_tools/cal_tools/tools.py

@@ -12,6 +12,7 @@ from typing import Optional
 from urllib.parse import urljoin
 
 import dateutil.parser
+import h5py
 import ipykernel
 from metadata_client.metadata_client import MetadataClient
 from notebook.notebookapp import list_running_servers
@@ -246,7+247,7 @@
 
     """
     directory = Path(directory)
     proposal = int(directory.parent.name[1:])
 
     try:
         run_info = get_run_info(proposal, run)
@@ -264,7+265,7 @@
         try:
             dates = []
             for f in directory.glob('*.h5'):
-                with h5py.File(f) as fin:
+                with h5py.File(f, 'r') as fin:
                     cdate = fin['METADATA/creationDate'][0].decode()
                     cdate = datetime.datetime.strptime(cdate, "%Y%m%dT%H%M%SZ")
                     dates.append(cdate)
             return min(dates)
-        except IOError:
+        except (IOError, ValueError):
             ntries -= 1
 
     msg = 'Could not get the creation time from the directory'