Minor additional features

`GdxReader`'s `get_parameter` now returns value directly if object is a scalar. Loading of data for parameters is now optional as is output dumping. Added `smax` function.
Added presolve assignments

Presolve assignments with possible conditionals for elements.
Added temp dir create function

Function to create a temp dir to use as work dir was added to utils.
Make GdxReader a context manager

GdxReader class can now be used with the 'with' statement. Can also be used with open/close functions.
Update examples

Examples updated to use GdxReader as a context manager and to use temporary director for work_dir.