Allow a supplied initial distribution of wealth in the transition

[arihantlodha-cmd]

The time path solver initializes the t=0 distribution of household wealth from the steady-state distribution, scaled so its aggregate matches the steady-state B. That assumes the economy starts already at its steady-state age-wealth profile. For a country whose model start-year age structure is far from its steady state, such as a rapidly aging population, that profile is a poor initial condition and can leave a small resource-constraint residual in the first periods of the path.

This adds an optional hook to supply an observed initial distribution instead. A new boolean use_initial_b_dist gates a new SxJ array initial_b_dist; when the flag is set, get_initial_SS_values takes the shape of the initial wealth distribution from initial_b_dist rather than the steady state. The aggregate is still rescaled to the steady-state B, so only the cross-sectional shape is overridden. The default leaves the flag off and reproduces the previous initial_b exactly, so existing runs are unchanged.

Adds test_get_initial_SS_values_custom_dist covering the aggregate-preserving and shape-override behavior; the existing test_get_initial_SS_values regression cases and the parameter/util tests pass unchanged.