new eval_bernstein_basis() in C in sfepy/discrete/iga/extmods/

- new sfepy/discrete/iga/extmods/nurbs.[ch]
- new sfepy/discrete/iga/extmods/cnurbs.pyx
- new setup.py files
support DEBUG_FMF define in sfepy/discrete/iga/extmods/setup.py
update IGDomain.__init__() to have nurbs.cs in FMField compatible shape
new ravel_multi_index(), unravel_index() in C
new eval_nurbs_basis_tp() in C
fix ravel_multi_index(), update unravel_index(), add to header file
new array2puint2(), array2puint1() helper functions
change return value of compute_bezier_extraction_1d() to 3D array
change nurbs.degrees dtype to int32 in IGDomain.__init__()
rename cnurbs.pyx -> igac.pyx
new eval_mapping_data_in_qp(), eval_variable_in_qp() in C

- use in sfepy/discrete/iga/mappings.py
new eval_in_tp_coors() in C
update NurbsPatch.__call__() to use eval_in_tp_coors()
docs: sync module index of developer guide with current sources
Merge branch 'iga-c'