Added support for persisting IndexedReads index to disk

[Prakash2403]

I recently came across a situation where I needed to persist the IndexedReads index to disk. I was successfully able to do it and wanted to contribute this feature to this repository.

Steps on how to store/load the index

1. How to store the index to the disk

>>> from pysam import AlignmentFile, IndexedReads
>>> bam: AlignmentFile = AlignmentFile('examples/sample.bam')
>>> name_index: IndexedReads = IndexedReads(bam)
>>> name_index.build()
>>> name_index.store('/some/path/to/sample.json')

2. How to load the index from the disk

>>> from pysam import AlignmentFile, IndexedReads
>>> bam: AlignmentFile = AlignmentFile('examples/sample.bam')
>>> name_index: IndexedReads = IndexedReads(bam)
>>> name_index.load('/some/path/to/sample.json')