Fix errors reported by lgtm.com

[jlaehne]

The code checker lgtm has a number of errors, warnings and recommendations for HyperSpy. We should use that to consolidate our code:

https://lgtm.com/projects/g/hyperspy/hyperspy

[jlaehne]

lgtm.com definitely includes some false-positives, but also identifies some real issues or chances for better code consistency.

For LumiSpy, I have now activated PR commenting by lgtm - takes a fairly long time (even though the project is still small), but can help to find some code inconsistencies that might be missed by manual review.

[jlaehne]

I reported three types of false positives to LGTM that affect us:

• undefined exported names imported by function: LGTM.com - false positive - missing names imported by function github/codeql#9642
• unused import to build API: LGTM.com - false positive - unused import needed for API github/codeql#9651
• unused import in try statement: LGTM.com - false positive - unused import in try statement github/codeql#9652