v0.3.4

Use mypy for type checking (#249)

* add typechecking with mypy

* typecheck packages without __init__

* reformat code

* add mypy to lock file

* resolve circular dependencies and fix bugs

* refactor

* add typecheck to CI

* change initial value to None

* test if CI passes without typecheck

* add type check to CI

* git status to CI for more output

* ignore mypy cache

* reformat code

* add assertion for non optional parameter

* remove redundant imports