make SuperLU as the default linear solver

[wgyang]

csparse occasionally fails. SuperLU is generally more robust and faster than csparse.

[osmsc]

@wgyang - it might not be possible to build superLU on windows because it requires non-standard system libraries. Under what conditions does csparse fail?

[wgyang]

@osmsc I see. OK, we can keep csparse as the default. Usually, cspare is fine. For some extensive arterial tree models, it may fail.

[caseyfleeter]

@wgyang should this issue still be open or can we close it?