Scansessiontool

* reorganized code for better maintainability
* added unit tests for automated testing
* improved documentation

---
Changes from previous dev:
* fixed bug loading help.txt
* fixed `scansessiontool` command

* reorganized code for better maintainability
* added unit tests for automated testing
* improved documentation

- fix for multi-echo data
- fix for BV links
- added (manual) software tests

- name fix

- changed package name to ScanSessionTool

- Fix for TBV links
- Scan Session Tool can now be installed with pipx