The used modules were not detected correctly on travis probably due to
some settings for the cd option. This was harmless since the list of used
modules was not checked indeed.

required fixing some incorrect escape sequence in plumedcheck

closing files to avoid too many open files

Fixes #493

modules should be open source

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

@gtribello I have to revert this since it fails on travis.
Can you please fix it and add it again on v2.4 (then merge, as you did)?

Thanks!

This reverts commit ce4bbf4e.

- Allow Plumed objects to be explicitly finalized
(can be used to make sure files are closed correctly)
- Allow Plumed objects to be used in context managers