Test for restarting swi_error_prop with gain_in

[Adeaem]

Write a test for swi_error_prop() where it calculates swi from 2 consecutive ssm input arrays, writing gain_out and restarting the calculation. Similar to https://github.com/TUW-GEO/pyswi/blob/independent_swi_calc/tests/test_swi_ts.py#L89

[Adeaem]

test also against a calculation from undivided input time series, not just hardcoded input.