Adds option for adding a temporary path

[havok2063]

Closes #48. This PR adds a new method to the Path, add_temp_path, which allows the addition of new temporary path templates into the local Python environment for use in sdss_access. This can be useful when developing new paths in code, when using sdss_acces and a tagged version of tree. This lets you add the paths in the interim until they can get properly added into the tree product and a new tag made. They are not persistent and only work for local path stuff only.

You specify a template name and path. The path must start with an environment variable and if it doesn't exist in the local environment already, you can define it with the envvar_path keyword.

from sdss_access import Access

access = Access()
access.add_temp_path("newCalib", "$LVM_SPECTRO_REDUX/{drpver}/@tilegrp|/{tileid}/new_calib.fits")
access.full("newCalib", drpver="1.0.0", tileid="1234567")
'/Users/Brian/Work/sdss/sas/sdsswork/lvm/spectro/redux/1.0.0/1234XX/1234567/new_calib.fits'

# add with a temporary envvar
access.add_temp_path('testFile', '$TEST_DIR/test_file_{ver}.fits', envvar_path='/tmp/test/dir')
access.full('testFile', ver='1.0')
'/tmp/test/dir/test_file_1.0.fits'

[joelbrownstein]

Looks like a useful feature!