minor, support nanosecond durations

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, read nanosecond durations in dt_strpdtdur() (with n or ns prefix)

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, pass on nanosecond durations in dt_strpdtdur()

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, evaluate and use nanosecond slot in dt_tadd()

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
test, provide regressions for nanosecond addition

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'feat/nanoseconds'

* feat/nanoseconds:
  test, provide regressions for nanosecond addition
  minor, evaluate and use nanosecond slot in dt_tadd()
  minor, pass on nanosecond durations in dt_strpdtdur()
  minor, read nanosecond durations in dt_strpdtdur() (with n or ns prefix)
  minor, support nanosecond durations