Choose new package name and class names

[LuzieH]

• maybe for the package PyTPT? (alex' proposal) -> and then rename transitions and src/transitions accordingly, also rename all the import statements and the folders in the git ignore file
• maybe also rename the classes (and the .py file they are in) to tpt_stationary, tpt_finite, tpt_periodic?

[axsk]

considering the classes themselves, i would call them just stationary, finite and periodic:

import pytpt as tpt
tpt.stationary()