Deal with files with large numbers of invalid frames

[mhvk]

@00rebe wrote:

I found another file that throws out the error: AssertionError: problem loading frame set 7966.
Cannot find header nearby.

fh = baseband.vdif.open('/mnt/raid-project/gmrt/rlin/ek036b/data/ir/ek036b_ir_no0002.m5a')
fh.seek(15360246)
fh.read(5 * 5 * 5 * 5 * 2**10)

This file has a large number of filled-in frames, in which the header has invalid_data=True, the correct frame size, all 0 otherwise in the first four words (so that nchan, seconds, etc., are wrong), and 0x11223344 in the second four header words as well as in the whole payload (i.e., the Mark 5B invalid data fill pattern). Specifically,

In [30]: fh.fh_raw.seek(7967*fh.header0.frame_nbytes)
Out[30]: 63990944

In [31]: f = fh.fh_raw.read_frame()

In [32]: [hex(i) for i in f.header.words]
Out[32]: 
['0x80000000',
 '0x0',
 '0x3ec',
 '0x0',
 '0x11223344',
 '0x11223344',
 '0x11223344',
 '0x11223344']

[mhvk]

In principle, the current machinery can sort-of deal with it, but it would help to have a way to mark frames as impossible to reach, maybe in RawOffsets?