Misc blech_post_process tweaks and issues

[Mraymon5]

I was running into some issues trying to run blech_post_process with the spreadsheet input. Some of these issues I think I resolved, some of them are still open.

Mainly, if I ran it with the spreadsheet once, then tried to run it again on the same sheet, it wouldn't run at all, because it was getting hung up on the reset_index command in blech_post_process_utils; the first time through it adds the column "level_0", and on any subsequent run, it tries to add it again but can't.

Then, I wanted to make the Split column of the spreadsheet a live input, so that it would read the split number in the sheet and use it, which required some additional modifications. Now, it still lets you pick how many splits you want in-program, but it also reports and defaults to the value in the sheet.

The last problem is one that I haven't figured out;
When I run the program multiple times, which I've ended up doing in the course of debugging, it just keeps appending the identified units to the hdf5 file, until ultimately that was saying that I had like 180 cells, but they were all just duplicates of the same 15 cells.

I dunno if the answer to this is, at least in the course of typical analysis, is just "don't run it multiple times", or if there's a way to wipe the saved units each time it runs so that you get a fresh slate?