Return the block header at the given height. If cp_height is zero, return the raw block header as a hexadecimal string. Otherwise a dictionary with the raw block header, the merkle root of all headers up to and including cp_height, and the merkle branch of header up to root.
See https://electrum-cash-protocol.readthedocs.io/en/latest/protocol-methods.html#blockchain-block-header
TODO: functional test