support to use dot as timezone offset separator

[biuiko]

Summary of changes

This PR adds support for using a dot . as a separator for timezone offset, like +9.5 means +09:30. Currently, the library only supports using a colon : as the separator for offset, but some users prefer using a dot for consistency with other time-related formats.

Changes

To achieve this, I have made the following changes:

• update parsing logic: add function scan::timezone_offset_with_dot_separator to calculate offset
• add specifier %.z to formatting and parsing

[pitdicker]

Fractional hours as timezone offset. That is a terrible format. Where did you encounter it?

[pitdicker]

Thank you @biuiko for contributing to chrono.

ISO 8601 is the prime example of a format that supports fractional values for components of a time or duration. Both are not (yet) supported by chrono. But even that format does not suggest using fractional values for an offset.

In #1083 I already had to make our parsing code for offsets quite a bit more complex to handle real-world cases. (Should yet that ready for review again...) Adding this as another piece of complexity needs a very good use case, such as a common library that generates such strings or some standard.