[hail] table tail

[iitalics]

closes #7357

• implements .tail function on tables:

def tail(self, n) -> 'Table':
    """Subset table to last `n` rows.

    Examples
    --------
    Subset to the last three rows:

    >>> table_result = table1.tail(3)
    >>> table_result.count()
    3

    Notes
    -----

    The number of partitions in the new table is equal to the number of
    partitions containing the last `n` rows.

    Parameters
    ----------
    n : int
        Number of rows to include.

    Returns
    -------
    :class:`.Table`
        Table including the last `n` rows.
    """

    return Table(TableTail(self._tir, n))

• refactored some of the logic in TableHead, because a lot of the behavior is the same

• specifically, moved some partition-counts calculations to is.hail.utils.PartitionCounts

[catoverdrive]

can you add a python test that does something to the effect of:

t = hl.utils.range_table(100, 5)
t = t.annotate(idx2 = hl.scan.count())
result = t.tail(10).collect()
expected = [hl.Struct(idx=i, idx2=i) for i in range(90, 100)]
self.assertEquals(result, expected)

[danking]

FYI, the PR shows up in my scorecard.hail.is queue if I'm an assignee.

[danking]

This looks awesome, a couple small comments.

I think there's an obvious extension here to TableInterval(l, r) which handles the head case when l=0 and the tail case when r=0 but also permits things like skip rows! I'm excited to design that after this PR lands!

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