Add Highs_presolve and Highs_getPresolvedLp to C API - and Highs_getLp and Highs_clearIntegrality

[jajhall]

[jajhall]

Also need Highs_getPresolvedNumCol, Highs_getPresolvedNumRow and Highs_getPresolvedNumNz

[jajhall]

Might as well add Highs_getLp for LPs and MIPs

[jajhall]

Trivial to add Highs_getLp, saving non-LP formulators from having to avoid Hessian data

When wanting to form the relaxation, it's currently only possible to change all integrality to HighsVarType::kContonuous, so add Highs_clearIntegrality and Highs::clearIntegrality

[jajhall]

Closed by #1574