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

@@ -231,19 +231,18 @@ def get_run_info(proposal, run):
 
 
 def get_dir_creation_date(directory: str, run: int,
-                          use_dir: Optional[bool] = False,
                           verbosity: Optional[int] = 0) -> datetime.datetime:
     """
     Return run start time from MyDC.
     If not available from MyMDC, retrieve the data from the dataset's metadata
-    in [directory]/[run], or from the folder on disk if `use_dir` is set.
+    in [directory]/[run] or, if the dataset is older than 2020, from the
+    directory's creation time.
 
-    If the files are not available, this function will raise a ValueError.
+    If the data is not available from either source, this function will raise a
+    ValueError.
 
     :param directory: path to directory which contains runs
     :param run: run number
-    :param use_dir: get the creation time from the directory, needed for older
-                    datasets.
     :param verbosity: Level of verbosity (0 - silent)
     :return: (datetime) modification time
 
     """
     directory = Path(directory)
     proposal = int(directory.parent.name[1:])
@@ -265,3+264,3 @@
     ntries = 100
     while ntries > 0:
         try:
-            if use_dir:
-                return datetime.datetime.fromtimestamp(directory.stat().st_ctime)  # noqa
             dates = []
             for f in directory.glob('*.h5'):
                 with h5py.File(f, 'r') as fin:
@@ -274,13 +271,10 @@ def get_dir_creation_date(directory: str, run: int,
                     cdate = datetime.datetime.strptime(cdate, "%Y%m%dT%H%M%SZ")
                     dates.append(cdate)
             return min(dates)
-
         except (IOError, ValueError):
             ntries -= 1
-        except KeyError:
-            msg = ("It seems that you're working on old data. Try setting "
-                   "`use_dir=True` to get file creation date")
-            raise ValueError(msg) from None
+        except KeyError:  # The files are here, but it's an older dataset
+            return datetime.datetime.fromtimestamp(directory.stat().st_ctime)
 
     msg = 'Could not get the creation time from the directory'
     raise ValueError(msg, directory)