SequenceRecord.__getitem__ should support slicing for certain BAM tags.

[rhpvorderman]

As mentioned in #129 tags that are directly related to sequence content such as methylation and signal should be properly trimmed as well. An implementation can be found here https://github.com/nanoporetech/dorado/blob/c3a2952356e2506ef1de73b0c9e14784ab9a974a/dorado/demux/Trimmer.cpp#L127.

This way the information can be passed on properly to minimap2 and other mappers.