Add function to write templates directly to file

[msto]

I've written something along the lines of the following for a couple client projects, and think it would be sensible to have this functionality available in fgpyo.

Ideally the syntax would parallel pysam's, e.g. writer.write(template), but I don't think this is possible without monkey patching AlignmentFile. Is there a preference between implementing this as a standalone function, a la below, or as a method on Template?

def write_template(
    template: Template,
    writer: AlignmentFile,
    primary_only: bool = False,
) -> None:
    """Write the records associated with a Template to file.

    Args:
        template: The Template to write.
        writer: An open, writable AlignmentFile.
        primary_only: If True, only write primary alignments.
    """

    if primary_only:
        rec_iter = template.primary_recs()
    else:
        rec_iter = template.all_recs()

    for rec in rec_iter:
        writer.write(rec)

[tfenne]

I agree this would be nice. I think I would prefer it as a function on Template, perhaps called def write_to(self, writer: AlignmentFile, ...). I think if it's an independent function it will be harder to find/discover.

[msto]

Addressed in #62