DPL: allow narrowing of large joins

[ktf]

This allows narrowing of large joins when they are not strictly necessary, allowing to simplify iteration and reducing the scope of helper functions and alikes.

[ktf]

@aalkin what do you think of this?

[aalkin]

I'm not sure as to what this is going to achieve. The iteration is not a heavy operation, the actual calculation of the pointer happens only when a column getter is called. The helper functions as a rule consume references to tables, so using this one would instead pay to construct a new (if lighter) set of iterators.

The only usage I can think of, are non-templated helper functions. If we implement narrowing as a conversion operator in the Table/Join class, then it would be possible to have helper functions, that require only part of a join, defined out-of-line with a concrete type.

[alibuild]

Error while checking build/O2/fullCI_slc9 for 13071e7 at 2026-06-10 16:11:

## sw/BUILD/o2codechecker-latest/log
100% tests passed, 0 tests failed out of 1


## sw/BUILD/O2Physics-latest/log
CMake Error at /sw/slc9_x86-64/CMake/v4.1.4-2/share/cmake-4.1/Modules/CMakeTestCCompiler.cmake:67 (message):
    ninja: build stopped: subcommand failed.

Full log here.