Duration from str does not account for negative sign

[ChristopherRabotin]

When durations are represented as string, they may be negative. As a user, I expect these to be valid when passed to the from_str function (or to the initializer in Python). However, that leads to an InvalidTimezone error.

For example, from python:

>>> Duration("-1 h 6 s")
---------------------------------------------------------------------------
HifitimeError                             Traceback (most recent call last)
Cell In[12], line 1
----> 1 Duration("-1 h 6 s")

HifitimeError: InvalidTimezone, invalid timezone format [+/-]HH:MM

This issue should also export the neg function to Python. Until then, the only way to negate a duration in Python is by the unnecessarily complicated: Duration.ZERO() - Duration("1 h 6 min") or Unit.Second*(-Duration("1 h 6 min").to_seconds()).

Finally, the from_str function should also support the UTF8 characters used in the formatting, like the micro:

In [32]: value_str = "-54 ms 102 μs 616 ns"

In [33]: Duration(value_str[1:])
---------------------------------------------------------------------------
HifitimeError                             Traceback (most recent call last)
Cell In[33], line 1
----> 1 Duration(value_str[1:])

HifitimeError: UnknownOrMissingUnit, unknown unit