Ensure that multiple investment years are interpreted properly in inputs/outputs

Currently, when interpreting the values of build decision variables, we are assuming that they are cumulative. I.e., if investment years are `x` and `y` where `x < y`, then `new_capacity(x) <= new_capacity(y)`. Therefore, for each investment year, we can create a grid representing the capacities in that year as `existing_capacity + new_capacity(x)` or `existing_capacity + new_capacity(y)`. See #97, #104 for more context.

However, if investments are not cumulative, then there is no relationship `new_capacity(x) <= new_capacity(y)` and the grid capacities in each investment year need to be interpreted instead as `existing_capacity + sum(i <= x, new_capacity(i)`.

The primary thing I think needs to be checked is the interpretation of the outputs, but during testing of this functionality we may find that there is additional work needed for the inputs as well, particularly relating to the `timepoints` parameter of `switchwrapper.prepare.prepare_inputs` and the information provided by the user in the `switchwrapper.grid_to_switch.get_inv_periods` function.