Feature: add single thread option

[mikeheddes]

I added the PIRANHA_SINGLE_THREAD option to CMake and I updated the documentation of some functions to note that the behaviour will deviate when configured with the PIRANHA_SINGLE_THREAD option.

I'm not sure how to update the settings.hpp file, I made the functions do noting or return a hardcoded value to still pass the tests but this may not be the best solution.

Closes #147

[bluescarni]

@mikeheddes thanks for the PR!

The travis CI has bitrot and I am not much inclined to try to fix it. If this PR solves your use case, I am inclined to just merge it.

I will then do a couple of maintenance bugfixes and release a new stable version of piranha with the single threaded option marked as experimental, if that works for you.

[mikeheddes]

@bluescarni there is no need to merge this PR now for my use case. I can use my forked repo to continue with my project. You can do the changes you are planning to do when you have the time for it. I will update my project to use this repo instead of my fork when you are done making the changes and released a new version.

Marking the single threaded option as experimental is fine. As long as I can use piranha single threaded I'm all good.