READ_WILLOW_HEADERFILE - Read file information from a Willow file
HEADER = READ_WILLOW_HEADERFILE(FILENAME)
Returns a structure HEADER with all of the information fields that
are stored in the Willow .h5 file FILENAME.
HEADER contains several substructures:
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fileinfo | Information about the file and its version
frequency_parameters | Information about sampling frequency
amplifier_channels | Information about amplifier channels
board_dig_in_channels | Digital input channels