feat: a `dosage` command for storing TR motif counts in PGEN files

[aryarm]

Note: This idea originated from the bottom of #73 (comment)

Once PGEN files can store multiallelic dosages, we should consider creating a command called dosage that can append dosage info computed using haptools' Genotypes classes to an existing PGEN file:

• It can use GenotypesPLINKTR to compute the genotypes, sum them to compute the dosages (and possibly divide by 2 so that they remain in the range [0, 2]?) and then write them to the existing file using PgenWriter.append_dosages_batch(). It should only change the PGEN file. The PVAR and PSAM files can remain the same
• If --out is specified, we should create a new PGEN file instead of overwriting the existing one. In the former case, we should also copy (or symlink?) the PVAR and PSAM files.

Once we do all of this, we should be able to use PLINK2 to analyze TRs! For example, we could do the following:

1. Convert a TR VCF into a PGEN file

   plink2 --vcf-half-call m --make-pgen 'pvar-cols=vcfheader,qual,filter,info' --vcf input.vcf --out output
   

2. Run haptools dosage to add dosages to the PGEN file

   haptools dosage output.pgen
   

3. Use the PGEN file as input to plink2. For example, we could use it with --r[2]-[un]phased, --ld-*, --pca, --glm, --freq, --clump, or --maf/--mac. Quoting from the documentation:

   Dosages are always used when present

[aryarm]

In Tara's script for computing the dosages, she uses the dosage values from Beagle, stored in the AP1 and AP2 fields. Could we extend the data.GenotypesTR library to handle those also?

Yes, in theory, we could just override the data.GenotypesTR._return_data method and have it load dosages from the AP1 and AP2 fields rather than using variant.GetLengthGenotypes()

And then we could implement the data.GenotypesPLINKTR.write_variants and data.GenotypesPLINKTR.write methods to handle writing the dosages

Would this be desirable in any way? Probably not. We don't do anything fancy in the data.GenotypesTR method that would make this faster. In fact, if anything, it would probably be more inconvenient (not less) because we would need to handle batching the writing of the PGEN file in chunks to avoid memory overruns.

[aryarm]

Update:
Melissa has actually been working on a TRRecord.GetDosages method which we could just call instead of TRRecord.GetLengthGenotypes on this line. That should make it super easy to add support for dosages in haptools. And once the annotaTR tool is implemented in TRTools, we probably won't need to try to write PGEN files with dosages anymore - but we might want to read them.