Add update_aux function to BAM records.

[ghuls]

Add update_aux function to BAM records as htslib now has the following functions:

bam_aux_update_str
bam_aux_update_int
bam_aux_update_float

[jch-13]

I was hoping that there is no need to implement bam_aux_update... functions because we can delete and recreate Aux fields—but I'll be happy to add them if there is a need.

[ghuls]

@jch-13 It might be worth to add as it can be faster than removing and appending an Aux field (especially if it can be updated in place).

See:
samtools/htslib#694