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This allows port "plumed +allmodules" to use DRR module.

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Macros that look for system libraries have been simplified in many respects:
1. Search for libs is now made with AC_SEARCH_LIBS, which does the same
   thing our macro was doing (try first with no explicitly linked lib, then try with
   libs) but is standard.
2. Actually, AC_SEARCH_LIBS is wrapped by PLUMED_SEARCH_LIBS that also
   follows --disable-libsearch correctly.
3. PLUMED_CHECK_CXX_PACKAGE has been added which simplifies searching
   c++ libs by providing a short test program (closes #274).
4. Both PLUMED_CHECK_PACKAGE and PLUMED_CHECK_CXX_PACKAGE now also
   set an env var that can be used later in the configure script
5. Both PLUMED_CHECK_PACKAGE and PLUMED_CHECK_CXX_PACKAGE accept multiple
   candidate libraries (similarly to AC_SEARCH_LIBS)
6. Blas, lapack, gsl, and molfile are now searched with PLUMED_SEARCH_LIBS

Got rid of first argument. Now, the name of the variable is computed
automatically from the option by replacing - with _

No change in ./configure file

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Add extended adaptive biasing force or dynamic reference restraining method

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Conflicts:
	CHANGES/v2.3.md

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When using --disable-libsearch, external molfile_plugins were unconditionally
disabled. Fixed now.

Added description of contributed modules

Improved text

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(should go with af0def1e)

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closes #244

Added cross refs in EDS and MAXENT

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Updates and Fixes to MaxEnt documentation

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closes #268

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manual: removed some incompleted and duplicated documentation from group.txt and expanded a bit that in MOLINFO
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