lochness.xnat to use XNATpy module instead of Yaxil

[kcho]

Yaxil module has been raising Accession Error for some sessions that didn't raise any issue when downloading using XNATpy. Replacing Yaxil to XNATpy module simplifies the lochness.xnat sync code and makes it easier to maintain the lochness repo. XNATpy is better maintained than Yaxil.

Referene to XNATpy

• https://xnat.readthedocs.io/en/latest/
• https://gitlab.com/radiology/infrastructure/xnatpy