Bugfix

Bugfix in selecting cells. Recent changes in the selection of cells that may intersect the domain introduce a side effect for "empty" elements in large array with level=0 and xp=0. Due to this new scheme, these empty elements were selected as leaf cells in domain... This is corrected with a test on level. A new function to compute the total number of leaf cells in a subset of CPU files has also been added to prevent the allocation of such large arrays with empty elements (when reading only a subset of CPU files, i.e. reading_method=hilbert). This should help minimizing the memory footprint of the code.