Fix size of dynamic derivatives (legacy representation) when there are unused exogenous and “nostrict” option is given

Those two settings were previously always at their default value for the static
model, independently of what the user would set in the .mod file.

Note that DynamicModel::determineBlockDerivativesType(), it’s legitimate to
replace max_{lead,lag} by max_endo_{lead,lag}, because for exogenous
lag=lead=0, and we no longer compute derivatives w.r.t. to endogenous that do
not belong to the block (so-called “other” endogenous).

Bytecode w/ block decomposition: no longer output derivatives w.r.t. exogenous and endogenous outside the block

Octave does not support “private” subdir inside plus folders (“+namespace”).

See https://savannah.gnu.org/bugs/?45444

As a consequence, and as a temporary measure, always output the
non-block-decomposed legacy representation.

Also drop the block kalman filter output, and drop now useless variables in
M_.block_structure.

Use std::filesystem::file_status and cached directory entries information to
avoid extra filesystem operations.

The block decomposed version is now under <BASENAME>/bytecode/block/.