Use a cache to store PySM3 data files

[ziotom78]

As it is often the case that PySM3 data files are not available due to network outage, this PR uses https://github.com/actions/cache to cache it. It should be quite efficient, as cache files are compressed using Zstandard, which in my tests showed very good compression ratios for PySM3 maps.

I use here the trick explained in the PySM3 User's Manual

[ziotom78]

Not relevant for the PR, but just to keep a record of my tests and to keep in mind the amount of storage we can save with different compression schemes. It's also a testimony to Zstandard's awesomeness!

I downloaded the PySM3 data archive and create a tarball:

$ git clone https://github.com/galsci/pysm-data pysm3-data
$ tar cf archive.tar pysm3-data

and then I compressed archive.tar using gzip, bzip2, and zstd. Here are the file sizes:

File	Size
Uncompressed	754M
BZip2	641M
Gzip	632M
Zstandard	615M

Zstandard is the winner here. However, the most impressive result comes from compression speed:

File	Compression time
BZip2	100 s
Gzip	34 s
Zstandard	3 s

Not only Zstandard has achieved the best compression ratio, but it has performed the compression in a fraction of the time required by the other two algorithms.

[ziotom78]

There is a problem here, because the cache gets saved and restored as desired, but it is never accessed when running the Mbs module.

After a few hours of debugging, I discovered that this happens because the environment variable PYSM_LOCAL_DATA (used to implement caching) is being overwritten in mbs/mbs.py.

@NicolettaK, if I understand correctly, you are using this feature so that we can pass our own CMB realizations to PySM3, is this correct?

[ziotom78]

@NicolettaK , may you please have a look at mbs.py, line 568? I would like to remove the line where the code changes the value of the environment variable PYSM_LOCAL_DATA, but I am unsure how to do so in a way that lets Mbs keep working as expected.

[ziotom78]

Testing the use of Path.absolute() after a suggestion in PySM issue #102.