More proper multiple zarr loading and bug fix for history length

[dfulu]

This PR includes two separate things I've been using locally

• First and most importantly, we were calculating the wrong history mins for the validation dataloader from the number of history steps the model requires. If history_mins=0 then the dataset returns a single satellite timestep at time t0 for the input X. This corresponds to history_steps=1.
• I think it is better practice to use the xr.concat() function for merging multiple datasets since we are only merging along a single axis. Plus previously we were not sorting by time which could be an issue if the user passed in yearly satellite zarrs not in order of increasing years

[phinate]

I just ran into this first one while testing things related to #65. Nice to have the fix already, looks good :)