fix: allow global error flag variable to be changed inside function

[ninsch3000]

Description

allow global error flag variable to be changed inside function. Added global keyword before variable inside the function definition, in order to allow the function to assign something to that variable. Additionally, changed test for nan to numpyesque call.

Fixes #136

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Conventional Commits guidelines

• I made sure the PR title follows the
  https://www.conventionalcommits.org/en/v1.0.0/

Checklist:

• My code changes follow the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes
• I have updated the project's documentation

[balajtimate]

I still seem to get the same error when running the workflow on some normal SE samples (other than the test samples). I think the issue is that the variable also gets assigned in the htsinfer_to_zarp function; adding global before variable inside that fixes it. Could you please look into that?

[ninsch3000]

Thanks a lot @balajtimate ! I tagged the e_flag as global in all functions now, hope this solves the problem.