Add warning when nominal_srate <= 0

copied from @jfrey-xx

Author: cll008

load_xdf.m

@@ -351,6 +351,7 @@
             if temp(id).srate > 0
                 temp(id).sampling_interval = 1/temp(id).srate;
             else
+			    warning('Nominal sampling rate of stream %s is 0. Calculated effective sampling rate might not be meaningful, relying on this rate is not recommended.', header.info.name);
                 temp(id).sampling_interval = 0;
             end
             % fread parsing format for data values