FlowAccumulator: add an area on-demand variable

[benbovy]

It might be useful to save drainage area as an model output diagnostic, even though an alternative method (e.g., using non-uniform surface runoff) used for computing flow accumulation.

However, it's difficult (not possible) to use both the DrainageArea process and another FlowAccumulator subclass in the same model.

Drainage area should be thus implemented in FlowAccumulator as an on-demand variable, so that it is available in all FlowAccumulator subclasses.