Reduce indentation by inverting if conditions

[troyraen]

Some of the spectroscopy functions have quite a few levels of indentation (almost 10 in some cases) due to nested for loops and if statements. This tends to squish the code into a narrow region on the right making it more difficult to read. This PR reduces some of the indentation by inverting the conditions of if statements where possible.

Simple example with just 2 levels (benefits are greater when there are more levels). This:

for sample in sample_table:
    table = query_archive(...)
    if len(table) > 0:
        # long block of code
        # ...
    else:
        print("no results found")

becomes this:

for sample in sample_table:
    table = query_archive(...)
    if len(table) == 0:
        print("no results found")
        continue
    # long block of code. now has more room horizontally.
    # ...

[bsipocz]

Some of these conditionals and loops are also very long, and for those the rule of thumb I was taught was to avoid any such section that is longer than one editor screen. (aka refactoring is advised if/when one ends in that situation)

[troyraen]

@bsipocz I totally agree, just trying not to spend that much time on this code right now. The changes I made here were some low-hanging fruit that will make the rest of my review a little easier. This tends to happen a lot with these notebooks, and I think it would be good for us to come up with a small number of relatively simple things the scientists can think about / do to avoid ending up in this situation with future code.

[bsipocz]

Yes, I totally meant it for that simple list of guidelines, rather than to fully rewrite of what we have. (OTOH, you practically did a couple of the refactorings in this PR, so we can even link to this as real life example of how to refactor a long conditional by e.g. inverting)

[bsipocz]

All looks correct, thanks!

[troyraen]

This code block (lines 50-62) is weird in two ways:

1. It seems to think the condition was if len(tab) > 1: instead of if len(tab) > 0: (which would make more sense given that the == 0 case was already handled).
2. The resulting variable tab_final is not used. Maybe the following line of code was supposed to use it instead of tab?

The diff in this PR makes it difficult to look at, and I'd rather not make substantive changes in this PR anyway. Opening an issue to be addressed separately. (#358)