Keep CrossSectionHelper.py up-to-date

[jabuschh]

As briefly mentioned in PR #249 I wanted to raise the topic of updating the values (cross section, branching fraction, etc) in the CrossSectionHelper.py.

I don't see a way to prevent checking the individual sources from time to time, which is obviously quite tedious...
As a first step, it might help to add a date from the last time these values were checked.

Maybe some has a better idea?
Please feel free to comment!

[MatthiesC]

In principle, we could just add a code comment as a first step... but on the other hand, we can just use the XSecSource field to give more details