Loguru: encapsulate loguru

Fixes: #647 (closed)

This attempts to encapsulate the dependency on Loguru by wrapping all its exposed symbols/macros and not including the luguru headers in VTK-m headers. At the archive/library level the Loguru symbols are not exported as per the fvisibility defaulting to hidden in VTK (I need to recheck this).

P.S. I am trying to move the changes of luguru library introduced here to upstream but it seems that the project is inactive (Last commit 1 year and half ago and several Pull requests ignored) Perhaps we should start thinking into moving to another logging library in the future.