update transform_integrals() - remove/ignore integral kind
remove kind attribute of Integral

- update Integrals.from_conf, .get()
- update Integral.__init__()
- remove Integral.get_key()
clean up sfepy/fem/integrals.py, add module docstring
update Term for no integral kind, remove .get_integral_info()

- remove PointTermBase
update sfepy.fem modules for no integral kind
allow 'i' as integral name in equations

- update create_bnf()
update examples for no integral kind
update tests for no integral kind
docs: update users guide and tutorial for no integral kind