Improve logic handling of nzeta and nzeta_from

[coderabbitai]

As discussed in PR #91, it would be beneficial to use a slightly different logic to handle nzeta and nzeta_from for future maintenance. See the conversation at this comment. Requested by @kirk0830.

[kirk0830]

due to the module has been refactored, this issue is not valid anymore

[kirk0830]

@coderabbitai You can close this issue now, because the read_input.py is not used for now