Added xdrfile to configure

Check if xdrfile library is installed.
Search is done by default and can be disabled with
--disable-xdrfile
Added --has-xdrfile option

This allows to conditionally execute regtests
Add method to retrieve path and mode

Can be useful if one wants to open externally a file
but generate the name with PLUMED rules
and properly use backups/append.
Implemented xtc/trr formats in DUMPATOMS

It can be triggered using xtc/trr file extension
or enforced with TYPE=xtc or trr. It is only available
when compiling against the xdrfile library.

Notice that files are properly backed up or appended
and suffixes for replicas are used as with gro files.
Regtest for xdrfile in DUMPATOMS

rt54-xdrfile is similar to rt54.
I changed the box shape in one frame to test that
non-cubic boxes are properly written.
Implemented xdrfile in Driver

Now driver can read using --ixtc and --itrr with the xdrfile library.
This is an alternative to molfile, and provides better support
for cells of arbitrary shape.
Added regtest for xdrfile in Driver
changed regtest so as to check new xdrfile driver

This is to test reading of xtc files from driver.
Notice that the added traj.xtc file contains
a generic cell and thus cannot be properly read with molfile
Properly append suffix to xtc/trr files.

Since now plumed can write xtc/trr files, in a multi replica
framework I think it makes more sense to add the replica suffix before
the extension. That is, with DUMPATOMS FILE=test.xtc, we get
files such as test.0.xtc and not test.xtc.0

The same happens for trajectories read from the driver.

Notice that this was already true for .gz files, and now applies
for .xtc and .trr files. All other extensions are still ignored
for backward compatibility.
Regtest for file suffixes

This is a regtest that also uses MPI to check that
files suffixes are properly treated when using trr/xtc
files. Notice that gro suffix is ignored for backward
compatibility.
Merge branch 'v2.1-fix-118'

This branch adds support for xtc/trr files using the xdrfile library
if found by configure script.

This allows for DUMPATOMS to write xtc/trr files. Additionally,
it is possible to read these formats from driver, as an alternative
to molfile plugin.

Notice that I added xtc and trr filename extensions to the list of those
for which the replica suffix is added *before* the extension. E.g., files
are written as "traj.0.xtc" and not "traj.xtc.0" to allow
easier file manipulation.

Closes #118
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