Add a setMinBufferRequirements() method to IOManager and BufferedIOHandler. This allows an application to specify its buffering needs. For example, when SNOWPACK runs grass detection, it needs the data up to 48 hours before the first time step to compute some averages. Using this call, SNOWPACK will request at least 48 hours of data in pre-buffering. if buff_before is set to something larger, it will be kept, but if it is set to something smaller, it will be increased.