datatype updates to a few nirfsg properties

[vnktshr21]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Changed frequency_settling property to be of type float instead of hightime.timedelta, since it can have multiple interpretation based on frequency_settling_units property value
• Changed interpolation_delay and relative_delay properties to be of type hightime.timedelta, so that it aligns with delay attributes across all drivers
• Changed exported_ref_clock_rate and ref_clock_rate properties to be of type float instead of enum, since the enums just represented raw float values

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

• Ensured successful build