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