add write and append to zarr

[jkingslake]

Add an option to sequentially write to a local zarr file each multi-burst xarray in the main loop in load.all and do not return the whole concatenated xarray.

This would be useful in cases when the dataset is too large to fit into memory and you can lazily load it later.

The edits would be around would be around line 274. Instead of adding the output of _all_bursts_in_dat_to_xarray to a list, we would write it to a zarr with the append_dim set to 'time'. e.g.,

multiBurstxarray.to_zarr('zarr_store_location', append_dim = 'True')