ensure trained_model is a Path before calling .suffix

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
Latest brainmapper fails because of a missing attribute when you call it with --trained_model.

What does this PR do?

Converts trained_model to Path before calling .suffix

References

Closes #470

How has this PR been tested?

Local manual tests with brainmapper by both @JRegg66 (thank you) and myself.
Local running of cellfinder tests.

Is this a breaking change?

No

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
  • we might have caught the bug if we improved tests on the brainmapper side Add more brainmapper tests brainglobe-workflows#70
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[niksirbi]

LGTM.