Add writePresolvedModel

[jajhall]

If users want to write out the model after the HiGHS presolve, they must extract it using getPresolvedLp, load it into HiGHS, and then call writeModel. Adding writePresolvedModel isn't difficult...

[feldmeier]

Alternative idea would be having write(filename) as a function of Model rather than the Highs class (and maybe read(filename) as a static function of Model producing a Model object).
The syntax would then be something like getPresolvedLp().write()

[jajhall]

Closed by #1741