Default data location about silx

[vasole]

I am more in favor of the first option (a data_dir specifying the directory containing the different data).

However, I would like to foresee a similar mechanism to the one used to install the different subpackages where each subpackage can be told the prefix to be used when trying to find its own data. Example module A that stores its data in directory A/data would be told that its data are to be found into data_dir/A/data

From my experience with Debian, we are going to need two variables: data_dir and doc_dir. They like to split module data and module documentation even if the module does not need the split.