Write function for determining the order of clean cooking access

[babakkhavari]

We want the user to give:

• a start year
• an end year
• a starting share of clean cooking
• a final share of clean cooking.

In between these years and shares we do a linear interpolation in order to gradually move towards the goal. This requires us to do an MCA in order to determine who gets clean stoves each year. This function should give a column stating which year the switch happens.

First thing to understand here is who switches first and if we can model it spatially.