Candidate for v4r7p6 release

This series of patches solves the issues which were discovered at the end of last year regarding the extraction of XML files needed to compute the statistics from LFNs on DIRAC log storage element. Furthermore, a new script was added to maintain functionality when XML files from CASTOR need to be accessed to generate statistics tables. Code was tested successfully on recent productions. A further development which I intend to release as v4r8 would be to disentangle the compatibility code branch (which downloads XML logs from CASTOR) as it became obsolete (see workaround).