required fixing some incorrect escape sequence in plumedcheck

Now it works like this:
- test- branches and pull requests are built with conda but not uploaded.
- tags are built with conda and uploaded with `tags` label.
- cron builds are built with conda and uploaded with `nightly` label.

Nightly builds are assignd a version number equal to branch.date
for version branches or 2.99.date for master branch, e.g.

2.5.190603
2.99.190603

In this way they always supersede the corresponding releases.

See #412

@tonigi

Requires adding a cpp-suppress

I made a few more changes to setup.py in order to work with MacPorts.
This allowed me to add a temporary Portfile for py-plumed
that is now used in travis tests.

The plan is to submit this Portfile to MacPorts after we release v2.5

Notice that one test is duplicated (python/test) in order to be available
when using nosetests from python.

disabled arrayfire from travis (but I kept the code if at some point we can make it numerically more stable)

this might decrease measured coverage, but will make the job
faster and possibly capable to finish on travis-ci

Notice newer cppcheck versions are slower. In order to make the check fit
on travis-ci I had to make a few extra changes:
- better optimization flags when building cppcheck
- force a number of ifdefs in the check

Now master takes approx 25 minutes, so we have room for adding new code.