Add indexunary helpers, break per-call agg import, document from_string - #604

…ons (gh-239)

gh-239 asks for value, row, column and index helpers in the indexunary
namespace, mirroring the four that select already has. This adds three of
them: indexunary.value(A > 3) returns the same expression as
A.apply(indexunary.valuegt, 3), with the same input validation, the same
lt-to-le and ge-to-gt thunk shifts, and the same errors as select's
helpers, adapted to name indexunary. The shared resolution logic moves to
core/operator/utils.py; select delegates to it and is unchanged.

index is deliberately not added, so this closes three quarters of gh-239.
indexunary.index already exists as an operator alias for rowindex, and is
used as one (v.apply(indexunary.index) and indexunary.index(v) are both
tested), so a helper function of that name would shadow a working
operator. Adding it needs a decision about that alias, which is a
maintainer call. Vector index comparisons are meanwhile reachable through
indexunary.row(v < k) or the indexle and indexgt aliases.

New matrix and vector tests compare each helper against the explicit
apply, and cover the BOOL result dtype, the thunk shifts, the scalar
expression path, and the error cases.

All eight operator-namespace from_string helpers (unary, binary, monoid,
semiring, select, indexunary, agg, op) had no docstring at all. Add
numpydoc docstrings describing the accepted string forms: a namespace
name, a dotted path such as numpy.mod, shorthand symbols such as + or >=,
and the [dtype] typing suffix. Each carries runnable examples.
test_from_string gains select and indexunary cases and asserts every
namespace's from_string is now documented.

Also fix a stale example in the indexunary from_string error message. It
offered 'row_index', which does not resolve; the operator is 'rowindex'.