Support for filterbank header with FREQUENCY_START instead of fch1 + foff

[loostrum]

Hi @scottransom,

The filterbank header format with FREQUENCY_START <list of freqs> FREQUENCY_END as output by e.g. SIGPROC's splice cannot be read by PRESTO. Maybe support for this could be added?

[scottransom]

Do you have an example file that you can post somewhere that uses that header functionality? If so, I can look at supporting it. Would probably be fairly simple (I think).