use separate linear solver instances in BatheTS
use separate linear solver instances in BatheTS

- update .__init__(), .create_nlst1(), .create_nlst2()
- allows factorization reuse without matrix digest (conf.use_mtx_digest = False)

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