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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.09%. Comparing base (f78a798) to head (cd740a1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2147   +/-   ##
=======================================
  Coverage   90.08%   90.09%           
=======================================
  Files          71       71           
  Lines       18643    18643           
=======================================
+ Hits        16795    16796    +1     
+ Misses       1848     1847    -1     

Flag	Coverage Δ
codegenunittests	84.44% <ø> (ø)
nidcpowersystemtests	94.65% <ø> (+0.04%)	⬆️
nidcpowerunittests	89.53% <ø> (ø)
nidigitalsystemtests	92.26% <ø> (ø)
nidigitalunittests	68.44% <ø> (ø)
nidmmsystemtests	92.72% <ø> (ø)
nifakeunittests	85.52% <ø> (ø)
nifgensystemtests	94.61% <ø> (ø)
nimodinstsystemtests	73.85% <ø> (ø)
nimodinstunittests	94.20% <ø> (ø)
nirfsgsystemtests	82.42% <100.00%> (ø)
niscopesystemtests	92.94% <ø> (ø)
niscopeunittests	43.20% <ø> (ø)
nisesystemtests	91.50% <ø> (ø)
niswitchsystemtests	82.03% <ø> (ø)
nitclksystemtests	94.87% <ø> (ø)
nitclkunittests	98.26% <ø> (ø)

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Files with missing lines	Coverage Δ
generated/nirfsg/nirfsg/session.py	90.30% <100.00%> (ø)

... and 1 file with indirect coverage changes

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