Also ran astyle

* Added R_POWER option to multicolvar/CoordinationNumber

This is to allow the coordination number moments from White and Voth
2014 JCTC and subsequent papers. They are used as a scalar proxy for
biasing radial distribution. Regtest and CHANGES line have been added as well
as the function/documentation.

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I used the same convention as in tutorial 1:

- plumedfile for plumed input
- verbatim with ">" for shell commands
- verbatim with "gnuplot>" for gnuplot commands

In addition, I used code{.py} to color python syntax

@gtribello just revert if you think there is some problem

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@gtribello in this manner it should appear at the beginning of the list

(master only)

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(v2.3)

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I added several examples with plumedfile, so I only
push this on master branch.

Related to #229

Fixed #229

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This ensures that steinhardt parameters are calculated correctly even if bond
vectors run parallel to the z axis

addresses #228

Allow passing a single trajectory to driver --multi.
The same logic is implemented as in IFile tool, that is:
- look for file.##.ext
- if it does not exist, look for file.ext

Here ## is the replica index and .ext the extension of the file

Fixed #228

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