[feature] : Tests for unit handling

[SylviaWhittle]

Is your feature request related to a problem?

Recently we had an issue where an .spm file needed the pm unit support in spm_pixel_to_nm_scaling . I added this in #84 but as @ns-rse points out, we don't have tests that show these are correctly handled.

Also we don't have m or cm. I am not sure whether these units will occur in .spm files but it can't hurt to add them.

Describe the solution you would like.

• Add tests for handling each unit type. Ideally without adding / finding new .spm files to run, we could use mocking to keep this simple.
• Add extra units just in case, m, cm.

Describe the alternatives you have considered.

No response

Sample Image

No response

Additional context

No response

[ns-rse]

Partially closed by #87