BaseData import from matrix format - allow paired label_start_col and label_end_col

[GoogleCodeExporter]

When importing a basedata set from matrix format the user specifies the start 
and end of the label section using the label_start_col and label_end_col 
options.  However, the system currently supports only one of each, taking the 
first start col and last end col (if the latter is specified).  

It would be useful to be able to specify pairs of start and end cols (with the 
last end col being optional).

The workaround at the moment is to use a label property table with either an 
exclude or exclude field.
http://code.google.com/p/biodiverse/wiki/DataStructures#Element_property_tables

Original issue reported on code.google.com by shawnlaffan on 27 Sep 2010 at 11:03