parseALTHeader

Helper function to parse out the ALT headers from the VCF file.

We need to do a bit more involved parsing in this function as we cannot use the .getOtherHeaders() call from HTSJDK. For some reason this only returns the first header when called, and we need all of them. The workaround is that we can get all the metadata, filter out any that are not ALT then parse the ALT header using normal string parsing. To make this easy, we just parse each piece of metadata into a key-value pair and then store in a map.