Implementation of unittests for vizualization functions + modification of load tests to run for Windows users.

In this branch the following were implemented:

• All vizualization tools now return the figure instead of simply plotting.
• Test functions have been implemented for all vizualization tools.
• The test_save.py and test_load.py files were modified so the test functions also pass on Windows.

requested review from @fima

assigned to @s184364

approved this merge request

mentioned in commit 1ebfb049

merged