provide reasonable grading function in gen_extended_block_mesh()
provide reasonable grading function in gen_extended_block_mesh()

- grading function now accepts x in [0, 1]
- update _spread_along_axis()
- verify merging by checking the number of nodes, update _get_extension_side()

sfepy/sfepy,lokik/sfepy,vlukes/sfepy,RexFuzzle/sfepy,rc/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,vlukes/sfepy,BubuLK/sfepy,rc/sfepy,lokik/sfepy,sfepy/sfepy,RexFuzzle/sfepy,vlukes/sfepy,rc/sfepy,lokik/sfepy,lokik/sfepy,RexFuzzle/sfepy,sfepy/sfepy,BubuLK/sfepy