Delay XRootD import to only require it if an eos file is requested

This MR delays the requirement of XRootD in utils.py. This allows Allen standalone to only require xrootd if reading files from eos, which is not really a requirement in most cases.

The motivation for this change is to allow macos compatibility in Allen, as there is no obvious procedure to install xrootd python bindings under macos.

Closes Allen#372 (closed)