2010-10-22 : fixed bug with variables using "grids" and singleton dimensions

2010-10-22 : upgraded to 2.4 in devel branch
2010-10-22 : exposed cmor_set_variable_attribute to users in Python
2010-10-22 : cell_measures var attribute is now called ext_cell_measures

Merge branch 'master' into devel

Conflicts:
	include/cmor.h