modernize pager code

[olaurino]

While testing the 4.10.0 conda binaries I ran into an issue with the utility function that displays information from sherpa in a pager:

sherpa/sherpa/ui/utils.py

Line 98 in 8e96fb6

def _send_to_pager(all, filename=None, clobber=False):

There is a lot of hard-coded logic that can be replaced with something more robust. I quickly gave a look around and found this:
https://stackoverflow.com/a/37585086

[olaurino]

Let me add that I don't think the issue is likely to happen at all in regular deployments... I am testing in barebone docker containers that do not have more installed and do not have PAGER set. Regular users usually have both. I worked around this in my tests by setting PAGER to less, which is present in the docker image I am using.

[DougBurke]

I was just having a discussion with a local user today who said that when they used a function like sherpa.astro.ui.show_model in a Jupyter notebook then the output when to the terminal running the server, and not the notebook. I suggested passing in a StringIO object via the outfile parameter and then displaying the contents, but it's not ideal.

[kglotfelty]

Adding to Doug's notebook example, a CIAO helpdesk user [ref 21322] is running sherpa via spyder and ran into the same problem that show_*() routines page the results into the terminal where the GUI was launched (or maybe into some system log if the GUI was launched from an Applications window)

[olaurino]

should we make this a higher priority?

[DougBurke]

I am okay with increasing the priority, but I haven't looked at your road map to see how this fits in. Note that there is a chunk of work we could do to make Sherpa work "better" within notebooks (e.g. #345 #347 but I haven't done an exhaustive check to see if there are more), but not for Sherpa 4.11 unless we get external contributors working on it.

I don't think it should a big job code-wise, but maybe more work on the policy/API-design side (i.e. what we want to happen and how to control things).

[DougBurke]

I've just tried out the pydoc.pager suggestion @olaurino pointed to and it seems to work in both a notebook (where there was no paging) and from IPython. So I think it's a worthwhile change.