Dysh data2 simplify data filenames

[teuben]

Although this will not be the final version, pending some discussion particularly on how to configure it in production time, there was a need to simplify filenames that work across being at GBO or on a laptop. The current solution is to provide an environment variable $DYSH_DATA within which directories (or symlinks) to places like example_data, acceptance_testing etc.
I'd like this merged into main sooner than later, since the nodding code testing (which has 10 examples) now depends on keeping filename easy to manage across sites.

[vcatlett]

Can you merge the latest version of main into this PR?

[teuben]

main is merged, only saw src/dysh/shell/shell.py and src/dysh/spectra/scan.py

[astrofle]

For what it's worth, we did remove wget from the dependencies some PRs ago. Since the code does not really depend on wget this does not seem like an issue.

[teuben]

the current dysh_data() still has some wget in it, but of course this would only work for actual fits files, not for datasets with multi-fits contents.

The example notebooks all are using from_url now. My problem with those is that it will often trigger a download, but i have my data in $DATA_DYSH, and so I'd rather have a frontend for this from_url function.

For example, in the PS example notebook we currently have

from dysh.util.download import from_url
url = "http://www.gb.nrao.edu/dysh/example_data/positionswitch/data/AGBT05B_047_01/AGBT05B_047_01.raw.acs/AGBT05B_047_01.raw.acs.fits"
savepath = Path.cwd() / "data"
filename = from_url(url, savepath)

and this would be replaced by

from dysh.util.files import dysh_data
filename = dysh_data(example="example_ps") 

though one can also use full example names like

filename = dysh_data(example="positionswitch/data/AGBT05B_047_01/AGBT05B_047_01.raw.acs/AGBT05B_047_01.raw.acs.fits")

[vcatlett]

Apologies for my delay; I just returned from some sick leave. It looks like there are now some merge conflicts which need to be resolved. Peter, can you take a look at those? I can help you make the changes if needed.