Update mapper policy file

[TallJimbo]

We should update the existing mapper policy file policy/genericMapper.paf with entries that match the data IDs we expect to use. We should also try to switch to the new yaml form for mapper policies at the same time. Because the MegacamMapper.paf file we started from seems to have a lot of stuff we don't need, I think it's easier to start over with the following steps:

• Copy the exposure.yaml and datasets.yaml from obs_base/policy into a single obs_generic/policy/GenericMapper.yaml file, with exposures: and datasets: headings.
• Remove any exposures/datasets that already have a filled in template (these are the same for all obs packages, so we don't need to customize them).
• Remove all entries besides template from the remaining exposures/datasets.
• Fill in the template entries with something containing %(visit)d, %(sensor)d, and %(source)s. I think the only datasets that you have left to fill in are ones that do correspond to sensor-level things not coadds.

After that, I think there will be a bit more we have to add to these files, but it should just be boilerplate (and I'm not entirely certain what it is).

[rbliu]

Should we also remove those exposure/dataset targets with the entry 'template: ignored'?