Fix build with GCC 14: drop const from vector_of_kll_sketches k_ and d_

[zeniel]

The copy and move assignment operators of vector_of_kll_sketches assign to k_ and d_, which are declared const. GCC 14 rejects this even though the templates are never instantiated:

src/vector_of_kll.cpp:146:6: error: assignment of read-only member 'datasketches::vector_of_kll_sketches<T, C>::k_'
src/vector_of_kll.cpp:147:6: error: assignment of read-only member 'datasketches::vector_of_kll_sketches<T, C>::d_'

As a result, the package cannot be built from source on any toolchain that ships GCC 14. That includes the Debian trixie based python:3.14 images, which is the failure reported in #65. It will probably also affect the Python 3.14 wheel builds proposed in #69. The root cause is the compiler version, not Python 3.14 itself.

This change makes the two members non-const, which keeps both assignment operators well-formed. There is no behaviour change.

I checked it on python:3.14-slim (Python 3.14.7, GCC 14.2.0), on both linux/amd64 and linux/aarch64:

• The package builds, and the existing test suite passes (37 tests).
• tdigest_double sketches serialized by this build are byte-identical to those from the official 5.2.0 wheel on Python 3.12, for the same input.
• Each build deserializes, merges and queries the other's sketches with identical quantiles.

Fixes #65.