Set `DATAPATH` for tests

[zonca]

Set DATAPATH for tests as an easily accessible variable somewhere, so it doesn't have to be re-defined in all the tests.

Originally posted by @DanielLenz in #489

[DanielLenz]

We could do something like:

E.g. in healpy/test/helpers.py

from pathlib import PATH

DATAPATH = PATH(__file__).parent.joinpath("data/")

and later

from .helpers import DATAPATH
d = fits.open(DATAPATH.joinpath('file.fits'))

[DanielLenz]

I've implemented this on https://github.com/DanielLenz/healpy/tree/datapath, but decided to go with the future-oriented (Python >= 3.4)pathlib module. Unfortunately, this won't work with Python 2.

We could simply stick to os.path.join(BASEPATH, filename) instead of BASEPATH.joinpath(filename) for now, of course, and re-visit this once we decide to drop Python 2 support.

[zonca]

we can also consider http://docs.astropy.org/en/stable/api/astropy.utils.data.get_pkg_data_filename.html

we already use it to download pixel weights on the fly

healpy/healpy/sphtfunc.py

Line 211 in fa31054

pixel_weights_filename = data.get_pkg_data_filename(