Support moving directly to a record by ID

[laroque]

Currently, ReadRecord accepts an offset indicating what record to read relative to the current record. There is not, however, support for an absolute indexing. This is made more difficult by the fact that the record ID of any given record may not be the same as its position in the file (record ID may have gaps), so there is not a reliable way to compute the relative step to get to a particular index (either record ID value or absolute within the file).

Options:

1. If nothing else, it would be nice to be able to explicitly go to the first record in the file. This would satisfy my current use case.
2. Implement ReadRecordInFile(int aRecordIndex), which does something equivalent to first going to the first record and then calling ReadRecord( aRecordIndex)
3. Implement ReadRecordId(int aRecordId) which tracks to a record by its recordID

[nsoblath]

(1) and (3) both sound like useful functions to have, and should be doable.

M3Stream has an index of acquisition IDs and record IDs in the form of: std::vector< std::pair< unsigned, unsigned > >.

I think we should probably use the existing M3Stream::ReadRecord() function to implement both of these, as there are a variety of important checks and accounting steps performed.

We can implement (1) if we know which record in the file we are on; we can just shift backwards by the appropriate number.

We can implement (3) by doing a binary search on the index vector using the record ID alone, which should be sorted in ascending order by construction. I think that an efficient way to do it would be to search between the current position -/+ the minimum amount (assuming continuous record numbers) and the beginning/end of the file.