Adding save_path to some examples

[raph-luc]

Addressing #25, adding the option to save to user-specified paths in some of the examples, also making various smaller fixes in attempts to get the examples to work on Fluent v231 and v232

As most of the examples do not properly kill running processes (e.g. using session.exit()), I have been changing them to show the gui when running (show_gui=True), also makes them easier to dev and debug, and I think having the gui works better for the examples overall. edit: see comments below, rather than using show_gui, added session.exit()s to the examples that were missing it

Related issues:
#28 - 'Ablation' still not working correctly
#29 - 'Separator-Collection-Efficiency' preliminary fix in this branch
#30 - 'Steady-Vortex-VOF' still not working correctly

Will continue to work through the examples as time allows in future branches/PRs

[mkundu1]

@raph-luc Thanks for fixing the examples for the latest version of Fluent. Few suggestions:

1. It is sufficient that the examples work only with the latest release of Fluent, which is 23.2 (will soon become 24.1). We don't need to maintain them for 23.1 or previous versions.
2. At some point we would like to execute the examples in the CI (Repo cleanup #26). show_gui will not work there. Also, most pyfluent usage assumes that Fluent is running headless.

[raph-luc]

As discussed with @mkundu1, some additional changes to how pyfluent by default handles CWD (particularly saving examples to CWD through download_file rather than EXAMPLES_PATH with implications for the CI testing) may be in order, to make it more intuitive for end users. Having so many examples with save_path may then be unnecessary, but I believe that would best be left for another issue and PR with bigger related changes, can easily trim down the examples later.