Github action to build and run template project on all 3 OS and make output folder

[drbergman]

This includes changes made in #241 (template project tests) and modifies the changes in #243 (make output folder). These are bundled together because of the intertwined history of those two PRs.

Unlike #243, this PR puts the output creation in modules/PhysiCell_settings.cpp to

1. put it out of sight for most users since they won't need to do anything with it
2. make it accessible to others not using the template project main.cpp

I also went ahead and simplified the code around reading the path to the config file to reduce redundancy, improve readability, and enhance maintainability.

[elmbeech]

sounds all good to me! I close PR #243.
thank you, Daniel!

[drbergman]

Some updates that will lead me to close this PR and open a new one (#249)

• remove the gh actions update to make room for @vincent-noel's matrix of builds
• create_output_directory() will now create the full path to the output folder (e.g. folder = output/subdir1)
• add an ArgumentParser class to more elegantly handle args; this also dovetails with another idea I had (see Passing in files and folders as command line arguments #220) to allow other files to be passed in as args rather than in the config file

[elmbeech]

This is already closed, but is there now output folder structure? with subfolders???
How far do this changes go?
This might cause heavily conflict with the physicell data loader!

As well, no longer having this information in the settings.xml.
I think this should be more discussed!
This might break a lot for downstream analysis!!!