Make the in-memory NPV cubes constructible from Python (#354)

[dazivo]

Fixes #354.

SinglePrecisionInMemoryCubeN and DoublePrecisionInMemoryCubeN raised AttributeError: No constructor defined - class is abstract on construction, on the released 1.8.15.0 and 1.8.16.0 wheels alike. Since NPVCube and AggregationScenarioData are abstract by design and JointNPVCube only joins existing cubes, no NPV cube could be built from Python at all.

The InMemoryCubeOpt overrides in orea_cube.i did not textually match the base's pure virtuals as the SWIG parser sees them, for two independent reasons:

• getT0/get dropped the base's = 0 default argument. SWIG expands a pure virtual with a default into one required signature per arity, so an override without the default satisfies only the full-arity form.
• the parameters were spelled QuantLib::Size/QuantLib::Real, which the SWIG parser cannot resolve: the using QuantLib::Size that would bridge them to the declared typedef size_t Size sits inside a %{ %} block the parser never sees (the same visibility mechanism as Python bindings: getCube/setCube declare different shared_ptr spellings, so in-memory cube reuse raises TypeError (regression in 1.8.16.0) #351).

Either mismatch alone keeps the class abstract (verified by bisection in a minimal SWIG module, table in #354); SWIG then treats the pure virtuals as unimplemented and drops the constructors.

The fix spells the overrides exactly like the base — bare Size/Real, with the base's default repeated on getT0/get — and declares %feature("compactdefaultargs") for getT0/get on NPVCube and InMemoryCubeOpt. The feature is load-bearing: without it SWIG emits a reduced-arity call like cube->getT0(i) for the default-argument overload, which does not compile, because the C++ overrides do not repeat the base's default and default arguments are resolved against the static type. It has to be declared for base and overrides alike, and before both classes, or the abstract check stops matching them.

Verified by building the bindings and running them in a debian trixie container (clang 19, swig 4.3, apt boost): both classes construct, values round-trip through set/get at either arity, and overload dispatch on a base-typed NPVCube proxy (index vs trade id, with and without the depth argument) resolves correctly. Warning profile unchanged, independent of the #353 alias fix (verified with and without it applied).

The accompanying test constructs both precisions and round-trips values through set/get; it fails with the abstract-class AttributeError against both released wheels.