re-evaluate Duration calculation

[nitram509]

Motivation

Re-evaluate how duration is computed and fix if necessary.

Hypothesis

We can calculate the time.Duration only when we know the date to which feel duration from iso 8601 applies to.
And even then we should use functions AddDate and Add to calculate it correctly.

See

feel/temporal.go

Line 270 in b5bde4e

func (duration FEELDuration) Duration() time.Duration {

Task

• check test code and add tests, if necessary, to cover variants of duration calculation
• verify hypothesis
• fix implementation were necessary