Add `force synthetic` option to load configuration

Description

Currently, the default configuration uses:

load:
    supplement_synthetic: true

Synthetic demand time series are used only when historical data are unavailable. However, for multi-decadal weather-year analyses (e.g. 1990-2020), it could be useful to have an additional option to force the use of synthetic demand across the entire study period.

Mixing synthetic and historical demand data introduces artificial discrepancies between years. Having a consistent demand modelling methodology makes it easier to attribute differences between years solely to meteorological variability.

Proposed solution

A configuration option such as:

load:
    force_synthetic: true

could provide a consistent methodology across all weather years while preserving the current default behaviour.

I already have a working implementation and would be happy to open a PR if the maintainers agree with this approach.

Start by locating the handling of load.supplement_synthetic in the configuration and demand-loading workflow, then check whether related configuration tests or example files exist. Add the force_synthetic option while preserving the current default behavior, and verify that synthetic demand is used across all weather years when enabled.