Store jars in user specific directory

[rnbguy]

Closes: #136

Description

Use a user specific directory to store jar files of the model checkers and tools, so that they are not downloaded multiple time when switching work directory.

We use ProjectDirs::from("systems", "Informal", "modelator")?.data_dir() from directories.

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For contributor use:

• If applicable, unit tests are written and added to CI. (N.A. Old tests takes care of this change)
• Ran go fmt, cargo fmt and etc. (or had formatting run automatically on all files edited)
• Updated relevant documentation (docs/) and code comments. (N.A. Doesn't change any API)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Re-reviewed Files changed in the Github PR explorer.
• Added a changelog entry, using unclog.

[andrey-kuprianov]

Looks good, thanks!