#16 block phase operator

[Steven Jenkins]

The last() operator is not useful if the INNER loop
doesn't have reliable timing. A better way would be to
create an snap_phas() operator that creates a phase of
length one with the latest snapshot available. Then the
tstart() operator could be applied to it for reliable
timing.

[Steven Jenkins]

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I meant sample(), not last().

[Steven Jenkins]

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Should create a fixed-length block as in the Ruby prototype.