Now that exceptions are translated, it is possible to dlclose an image
that is not used anymore.

Now we have explicit only used with C++11 and where it should be used.

Now every exception thrown within PLUMED is remapped in the wrapper.

In particular:
- All standard C++ exceptions are remapped to equivalent ones.
- PLUMED exceptions and lepton exceptions are remapped to ad hoc classes.
- Any other exception derived from std::exception is mapped to std::exception.
- Any other exception is rethrown after printing a warning.

I changed Plumed::invalid() to Plumed::makeInvalid() and added a new function
Plumed::makeValid(). These names are more inline with stdlib names
(e.g. make_shared), though with our conventions for capitalization.

Hopefully there's no code yet using this (beta) interface.

In addition, I made it possible to use a macro to change the behavior
of the default constructor to makeInvalid(). This might be
useful in python wrappers to avoid opening the library twice
(python objects always use the "nullary" - default - constructor first).

now covered by the regtest better

It should prepend the libplumed.so library with prefix -Wl,
so as not to disturb NAMD makefile. (suggested by @jcphill)

Should have been removed with 1221e3e1

cc: @dvdesolve

See #407