Use 3rd party logging library

[garth-wells]

Replace home-baked logging with a library, e.g..

• https://github.com/gabime/spdlog
• https://github.com/SergiusTheBest/plog
• https://muflihun.github.io/easyloggingpp/

See also https://neutronsharc.blogspot.co.uk/2016/12/choose-c-logging-library-im-recently.html.

[Opinions of the Boost logging library online are not very positive - heavy weight and slow.]

[jhale]

I'd suggest using spdlog as it now has pybind11-based wrappers: https://github.com/bodgergely/spdlog-python

Would it be best to include the spdlog headers in the DOLFIN source tree or have it as (another) dependency like Eigen?

[garth-wells]

See #311.

[garth-wells]

Merged in PR #311.

[jhale]

spdlog turned out to be very slow at compile time. Other options?

[garth-wells]

Would need to check compile speed for each.

• Boost.Log v2 (https://www.boost.org/doc/libs/1_69_0/libs/log/doc/html/index.html)
  We already depend on Boost, so that makes it easy. But, with C++17 we might at some point want to remove the Boost dependency
• plog (https://github.com/SergiusTheBest/plog)
• https://github.com/emilk/loguru
• https://github.com/KjellKod/g3log

[chrisrichardson]

Using loguru, as it is compatible with Google glog (as are many others), which should make it easy to replace should we need to.