Maya Plugin - Add C++ Logging Library

[david-cattermole]

Feature

Logging inside the mmSolver plug-in is handled very simply, with std::out and std::err.

The solver is getting larger, and we need a logging infrastructure to help us save log files to disk for all the different parameters. This will also help debugging, since we get ask users for log files.

After looking at many different C++ loggers, loguru seems to be a good choice for mmSolver.
loguru's license is Public Domain, which is compatible with LGPL (mmSolver).
loguru doesn't seem to be C++11 dependent, which is important because we still officially support Maya 2016 and 2017.
loguru seems simple to use. loguru does not need be compiled into a library to be used (header only or static library).

https://github.com/emilk/loguru

While implementing logging using loguru, if a problem is encountered, we will search for another logging library.

Another alternative library is Google's glog. https://github.com/google/glog

[david-cattermole]

I'm pushing this feature to v0.4.0 (or perhaps v0.3.x), as it's not critical for the features in v0.3.0.

[david-cattermole]

I'm not sure this is really needed. It's fairly low priority.