Add 'memb_dim' in MergeRefToExp

I open this issue thanks to @nperez. Now, when data1 and data2 have different member dimension, we get the following error:

> mer_exp <- CST_MergeRefToExp(data1 = past_obs, data2 = tas_exp,
+                              start1 = list(1,1), end1 = list(1,3),
+                              start2 = list(1,4), end2 = list(1,11))
Error in Apply(list(data1, data2), target_dims = time_dim, fun = "c",  : 
  Found one or more margin dimensions with the same name and different length in some of the input objects in 'data'.

To solve this, it could be added 'memb_dim' parameter to check this dimension.