Support writing RDD[Feature] to various file formats

[ryan-williams]

• GTF
• BED
• .interval_list (cf. Add ability to write .interval_list files #703)
• NarrowPeak

I'm imagining methods in FeatureRDDFunctions analogous to the {Sam,Bam,Adam,Fastq} methods in AlignmentRecordRDDFunctions.

[laserson]

+1, though this may run into some model incompatibilities.

[fnothaft]

Do we want this to be automagical or explicit or both?

BTW, I didn't realize you could do subfeatures with the checkboxes. Nifty!

[ryan-williams]

I like the place we netted out at with "reads" files/formats: expose a method that infers format from file format and delegates to other exposed format-specific methods, so that the caller can choose to specify format or not.

[fnothaft]

I think that sounds good too. Let's have that be the default approach moving forward.

[fnothaft]

Closed by 8abb47a.