Add the 4.4.2.4 DRB.UEActive measurement from 3GPP TS 32.425 version 17.1.0, which provides (simply put) the average number of active connected UEs per time period.

From what I understand, this addition constitutes the first non-counter statistic that xlte parses from enb.xlog stats messages: as such, some additional infrastructure is needed in the library to accomodate these cases, which I have added. These non-counter statistics are much simpler to handle than counters, as they are not cumulative and therefore do not need adjusting between different measurement periods.

This is currently a first, functional draft that needs some polishing, but the essence of the changes are here. It just needs to be cleaned up and made easily extensible to match the rest of the library.