Enable writing of output data in double precision

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In order to better control the numerical stability of the code, and to have 
better regression tests, we need to produce output files in double precision.

This is not really hard, as geotop already uses double precision floats for all 
the computation. The only changes to do are:

* in ``configure.ac``, to add a configuration option 
(``--enable-double-precision-output`` ?) to control the generation of a define 
``USE_DOUBLE_PRECISION_OUTPUT``
* in all the files that write output files, to use a different format ("%.12g" 
?)

This change should be done on both UZH and TN code, probably using `tn_exact` 
and `uzh_exact` branches.

Original issue reported on code.google.com by [e-mail] on 30 Sep 2013 at 9:40

[Google Code Exporter]

Original comment by [e-mail] on 30 Sep 2013 at 9:43