Refactor Hi L1a histogram

[subagonsouth]

Algorithm Description:

Due to changes in PR #687, the allocate_histogram_dataset function is probably no longer needed. Check the TODOs in the create_dataset function and refactor the code as you see fit.

Note from the PR:

The allocate_histogram_dataset function was really just my way of avoiding having an intermediate data storage. I wanted to go directly from the packet into the xr.DataSet arrays. With your new function, this is no longer achieving that goal. I will write a ticket to look at this Hi code and address the TODOs.

In addition to the above, this should also address the TODO on line ~60:

    # TODO: Look into avoiding the for-loops below
    #       It seems like we could try to reshape the arrays and do some numpy
    #       broadcasting rather than for-loops directly here. Ticket: #700