Added new ENABLE_XXX_FILTERING keys, code factorization in 1dinterpolations and new option for decimal dates in CSV plugin

The spatial_resampling, 1d interpolaitons and 2d interpolations have
been renamed to have more consistent naming.

This is necesary for the packaging (it is much beter to have an
"inishell-apps" folder in /usr/share than an "applications" folder)

When both a default value and precision=... where provided in the xml
file, the precision was reset to the default precision. Now the default
precision is reset to the XML provided precision.

In order to give the user the required flexibility to enter the numeric
values he/she wants to, precision specifications have been almost
everywhere in the xml files (a few places still rely on the default
settings because it makes sense). The units have also been added where
possible.

The virtual stations are given as "VSTATIONx", not "POSITIONx"!
Moreover, more links to the online documentation have been added and the
syntax highlighting (in the PreviewWindow) now supports the coordinates
specifications (otherwise, coordinates provided with ";" were shown as
comments).

Don't auto-select items in editable dropdown, give key as tooltip if no help available, minor MeteoIO XML changes

Fix reading a decimal number with more decimals than previously (it was only checked, not set to the widget). Add some MeteoIO XML.