MMS ValidMin value for Epoch times changed

In the newer example CDF files released (for example mms2_sdp_fast_l1b_20150410.cdf) the time variables have changed their MinValue to 2000/01/01 00:00:00:000000 (in TT2000). This is changed from previous example CDF files which had 1999/12/27 12:09:37 as their lowest valid value. These particular values are not described in any documentation I have found (only have the parameter listed as required by MMS_CDF_Format_Guide) but in order to be consistent amoung other FIELDS data product they are changed here as well.
Merge branch 'devel' of github.com:irfu/irfu-matlab into devel