feat: Add bedrecord/interval converters

[msto]

🙏

Let me know if you'd like equivalent methods added to Interval

[msto]

I ran into circular imports when the converters were declared on BedRecord, so I moved them to Interval.

I have a slight preference for putting them on BedRecord, but not enough to deal with the headache of restructuring the project

[codecov]

Codecov Report

Attention: Patch coverage is 92.68293% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 91.79%. Comparing base (9bbabbe) to head (7a9729c).

Files	Patch %	Lines
pybedlite/bed_record.py	75.00%	1 Missing and 1 partial ⚠️
pybedlite/overlap_detector.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   91.41%   91.79%   +0.38%     
==========================================
  Files           7        8       +1     
  Lines         489      524      +35     
  Branches       86       92       +6     
==========================================
+ Hits          447      481      +34     
  Misses         26       26              
- Partials       16       17       +1     

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[nh13]

Minor fixups that I trust you can make. Thank-you!!!

[nh13]

@msto looks good to me based on your comments!

[msto]

@nh13 Minor changes to address the circular import and standardize on from_FOO() constructors on both classes.

lmk if it still lgty

[nh13]

Can you just try the one suggestion, and if that doesn't work, ship it?

[msto]

for posterity

without importing Interval, both flake8 and mypy complain about the forward reference

pybedlite/bed_record.py:196:57: F821 undefined name 'Interval'
    def from_interval(cls: Type["BedRecord"], interval: "Interval") -> "BedRecord":

pybedlite/bed_record.py:196: error: Name "Interval" is not defined  [name-defined]