BUG: fix compatibility with gadget 2 files using '*.gad.*' template

[neutrinoceros]

PR Summary

This fixes an issue where some Gadget2 files can't be loaded because their names aren't compatible with the filename template we have.
This problem was reported on Slack by @Gustav-Madsen. Here's an example dataset to test this on http://skun.iaa.es/SUsimulations/UchuuDR1/ShinUchuu/SamplingParticle/snapdir_070/
I was able to make a simple plot of this dataset, only downloading the first 3 files

import yt

ds = yt.load("snapdir_070")
yt.ParticleProjectionPlot(ds, "z", ("all", "Mass")).save("/tmp/")

[chrishavlin]

Looks great! A couple of questions, but don't think they should hold up this PR, even if you agree:

GadgetHDF5Dataset._set_owls_eagle does something similar with filename_template -- is it worth moving your change into its own method at the GadgetDataset level (e.g., GadgetDataset.set_filename_template()) to use in both spots?

Related -- do you think there's any benefit to adding an explicit way to set the filename_template (maybe a kwarg)?

[neutrinoceros]

GadgetHDF5Dataset._set_owls_eagle does something similar with filename_template -- is it worth moving your change into its own method at the GadgetDataset level (e.g., GadgetDataset.set_filename_template()) to use in both spots?

I wasn't aware ! Yes, I think this is worth including in the present PR, thank you !

do you think there's any benefit to adding an explicit way to set the filename_template (maybe a kwarg)?

Clearly it would have a benefit that users that need such patches wouldn't need to either wait for a new release or install from source. On the other hand such changes should hopefully be extremely rare so I don't think it's worth the additional maintenance weight.

[neutrinoceros]

I spoke too soon. I don't feel confortable to change the _set_owls_eagle method after all

1. I'm not sure such a change is needed or makes sense
2. I don't think the _set_owls_eagle method belongs in the Gadget frontend in the first place (should probably be in owls, or eagle !).

Any way I don't understand these formats well enough to make further changes in that direction.

[chrishavlin]

Ya, totally fair. I obviously don't fully understand these formats either :) These are also simple enough to fix down the line if there is need. I'll go ahead and merge since this is a bug fix.