new use_mtx_digest parameter in ScipyDirect, new .clear()
new use_mtx_digest parameter in ScipyDirect, new .clear()

- update .__init__(), .__call__(), .presolve()
- new use_mtx_digest parameter in ScipySuperLU, ScipyUmfpack
- update presolve behaviour

rc/sfepy,rc/sfepy,rc/sfepy,sfepy/sfepy,sfepy/sfepy,vlukes/sfepy,vlukes/sfepy,sfepy/sfepy,BubuLK/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy