Now all installers use a temporary directory

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Actually this should have been done together with 21be029e

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this is to test FIT_TO_TEMPLATE with TYPE=OPTIMAL and
domain decomposition (see #214)

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This is a nasty workaround. Since FIT_TO_TEMPLATE needs to
access forces stored in Atoms, it is necessary that they are kept up
to date. With this fix, there is an extra function
allowToAccessGlobalForces()
that switch off an optimization that would otherwise make the
forces wrong when running in parallel

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(thanks to @valsson)

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Set pointer to NULL with plumed_finalize() (to avoid subsequent usage)

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addresses #213

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addresses #213

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Notice that there are a few errors in some formulas.
To see all the warnings, look in the raw log from travis ci.
I also opened an issue right now (#213).

Anyway, I checked and the same errors were in the previous manual
(with doxygen 1.8.12). I thus upgrade to latest doxygen.

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(reported by Omar Valsson)