[lmi] C++11 transition step 0: replacing 0/NULL with nullptr

[Vadim Zeitlin]

 Hello,

 I've made a pull request replacing all uses of 0 and NULL in the sources
with nullptr, please see https://github.com/vadz/lmi/pull/32

 As I wrote there, most of the work was done by clang-tidy and I just
reviewed its results and rebuilt (under all platforms, including using the
official makefiles) and reran the tests (the rounding one indeed fails, as
you recently mentioned, but no other ones seem to do, although I'm always
nervous about this because the test suite doesn't clearly state how many
tests have failed at the end, so it's easy to miss something, I'd like to
change this...). I did try to find any remaining occurrences clang-tidy
could have missed, but only found one of them. I could have missed some
myself too, of course, but I don't know of any way to ensure that this is
not the case and, even if I did, it's not catastrophic.

 Needless to say, this PR affects a lot of the code, so it would be nice to
merge it soon to avoid conflicts in the future, but, OTOH, seeing that 99%
of the job can be automated, it wouldn't be difficult to just redo it again
in the future neither. I'd also ask you to have a look at my two other
recent PRs (30 and 31) for the clang warnings in the tests as I currently
need to always apply them manually before running clang-tidy to let it do
its job.

 If this PR is accepted, I'll continue with the next C++11-ization step,
probably the use of "override" as it seems like the next least
controversial one.

 Thanks in advance,
VZ