[RFC] improve readability of time literals

[bkueng]

We have various expressions like:

if (hrt_elapsed_time(&start_time) > 8000000)

The number of 0's are hard to read, and we cannot use 8e6, since this is of type double.
C++11 introduced user-defined literals, which allows to write the following:

if (hrt_elapsed_time(&start_time) > 8_s)

As an example, I adjusted the land_detector to use the new literals. Commander would be a good candidate as well. We could also be more verbose if desired, for instance 8_sec or even 8_seconds.

I consider this as a start towards using more units throughout the code, for all the SI units we use, not only time. A potential library for this is https://github.com/nholthaus/units, as pointed out by @dagar . Some notes about it:

• Uses double by default as underlying type, but it can be customized
• requires the <chrono> header
• not specifically targeted towards embedded, but it's efficient as conversions are done at compile-time
• only very limited literal support, so it's kind of orthogonal to this PR / it will be simple to adjust later on.

Thoughts?

@MaEtUgR @RomanBapst @Stifael

[Stifael]

good idea.
the potential library is built on c++14 though.
What speaks against having our own operators?

[bkueng]

What speaks against having our own operators?

IMO nothing, but once we grow & extend it and add for example a meter class, it will be less effort to just use an existing and already tested library.

[MaEtUgR]

@bkueng You finally found a nice solution for the annoying trailing zeroes with all these microsecond values. Thanks a lot! I totally support the use of such useful language features.