Add now_local and now_utc for DicomDate, DicomDate and DicomDateTime

[Cryt1c]

Implementation of #691

1. Instead of using "today" for the date I decided to use "now" for all methods. Please let me know if you still wanna go for today.
2. I created a variant for the local time and for utc.
3. I am not sure how I should setup the tests here. Should I look into mocking the chronos library or do you have another idea on how to handle it?

Looking forward to your feedback!

[Enet4]

This is much appreciated!

1. Instead of using "today" for the date I decided to use "now" for all methods. Please let me know if you still wanna go for today.

OK, I just looked and even the today methods in chrono are deprecated, so we'd better not use them from the start!

2. I created a variant for the local time and for utc.

Makes sense. DICOM is particularly quirky here, because many key attributes are split by date and time, and the timezone cannot be written when this happens. There may be even more ways to assist users when working with DICOM dates and times, but offering these possibilities should be a step in the right direction.

3. I am not sure how I should setup the tests here. Should I look into mocking the chronos library or do you have another idea on how to handle it?

I think it's OK that we test this by issuing a now date-time via chrono to serve as a reference. Then for the date-time output we would check for differences with the reference so that they are within an acceptable margin (like 1 second or so).

[Enet4]

Thank you!