Write ADR-01 on overall tool architecture

The purpose of ADR-01 is to outline the decomposition of `Modelator` into components and their APIs, such that the work on the tool can be efficiently parallelized. As the tool will undergo many changes in the near future, the constructed document will have to also undergo several refinement iterations; please read the resulting document critically, and feel free to propose changes. The description of architecture of sub-components will be delegated to subsequent ADRs.