Xpath lxml python wrapper

[siwelwerd]

Previously, we used a command line tool called xpath to find, for example, all <activity>s that had a <answer> but no <statement> (#763), see also #512. This command line implementation had a perl dependency, and thus is not present in the Docker image we are using. @mitchkeller shared with me a couple scripts that use a python implementation of Xpath to do exactly this task (in this case, find all <image>s lacking a <description>; see #898), so they should run just fine in the Docker image.

I think I want to generalize Mitch's second script slightly since we have found other use cases for this previously, but it should not be hard to pass a couple things as a command-line argument and feed those into xpath within the python script.