`concatenate` is buggy with internal samples

[petrelharp]

As discovered in #3168, union is a node-wise union (not edge-wise) and so is not functioning how we imagined it was in implementing concatenate. For instance, the following fails:

t = tskit.TableCollection(sequence_length=2.0)
p = t.nodes.add_row(time=1, flags=1)
for _ in range(2):
    c = t.nodes.add_row(time=0, flags=1)
    t.edges.add_row(parent=p, child=c, left=0, right=2)
ts = t.tree_sequence()
ts1 = ts.keep_intervals([[0, 1]]).trim()
ts2 = ts.keep_intervals([[1, 2]]).trim()
cts = ts1.concatenate(ts2).simplify()
cts.tables.assert_equals(ts.tables, ignore_provenance=True)

Indeed,

>>> print(cts.draw_text())
1.00┊  0  ┊ 0     ┊
    ┊ ┏┻┓ ┊       ┊
0.00┊ 1 2 ┊   1 2 ┊

Passing in node_mappings should also fail in many situations.

[hyanwong]

Proposed fix at #3183 (comment) - this is a PR with a load more tests attached, but will need re-working.