updating qflag with gain_in

[Adeaem]

When restarting swi with gain_in and the ssm['sm'] starts with nan(s) the calculation will begin from the first valid ssm['sm'] index.
Qflags need to be updated up/down for each timestamp, therefore a loop is needed to downgrade the flag for each nan leading up to the first valid ssm value.
https://github.com/TUW-GEO/pyswi/blob/independent_swi_calc/src/pyswi/swi_ts/swi_ts.py#L97

[Adeaem]

if gain_in:
        if ssm['sm'][0] != nan and ~np.isnan(ssm['sm'][0]):
            qflag[0] = [1 + gain_in['last_qflag'][i] * np.exp(-(1. / float(t_value[i]))) for i in range(0, len(t_value))]
        else:
            qflag[0] = [gain_in['last_qflag'][i] * np.exp(-(1. / float(t_value[i]))) for i in range(0, len(t_value))]
        for f in range(1, j):
            if ssm['sm'][f] != nan and ~np.isnan(ssm['sm'][f]):
                qflag[f] = [1 + qflag[f - 1][i] * np.exp(-(1. / float(t_value[i]))) for i in range(0, len(t_value))]
            else:
                qflag[f] = [qflag[f - 1][i] * np.exp(-(1. / float(t_value[i]))) for i in range(0, len(t_value))]

Flags are updated for the first timestamp after restart with last_qflag stored in gain_in.
If the input ssm has no value for some time the qflag is updated up to when the swi calculation starts at j (j = 1 + first_ssm_idx).

This now works well, but not closing yet as this can be re-written more concisely perhaps.