I removed matheval everywhere in the code. Now, MATHEVAL
(and CUSTOM) keywords are automatically using lepton (see #244).

Notice that env var PLUMED_USE_LEPTON is now ignored.

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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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Pointers were replaced with std::vector. This makes the syntax cleaner.

I changed all the forward references to unique_ptr
using an auxiliary class (ForwardDecl).

Notice that in this manner the order of destructors
is exactly the reverse of the order of constructors.
This forces me to move a variable up in the layout
of PlumedMain to avoid a segfault.

This change is necessary to make plumed exception-safe.

This change is necessary to make plumed exception-safe

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closes #268