chore: Explicitly define where package files reside

[ns-rse]

The pyproject.toml was missing a [tool.setuptools.packages.find] section which meant that on trying to install
locally it for some reason tried to include a tmp/ directory (something I often create within projects for tasks that
need undertaking but is explicitly excluded from Git via .gitignore) and I observed the following complaint from pip

❱ pip install -e .
Obtaining file:///home/neil/work/git/hub/AFM-SPM/AFMReader
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... error
  error: subprocess-exited-with-error

  × Getting requirements to build editable did not run successfully.
  │ exit code: 1
  ╰─> [14 lines of output]
      error: Multiple top-level packages discovered in a flat-layout: ['tmp', 'AFMReader'].

      To avoid accidental inclusion of unwanted files or directories,
      setuptools will not proceed with this build.

      If you are trying to create a single distribution with multiple packages
      on purpose, you should not rely on automatic discovery.
      Instead, consider the following options:

      1. set up custom discovery (`find` directive with `include` or `exclude`)
      2. use a `src-layout`
      3. explicitly set `py_modules` or `packages` with a list of names

      To find more information, look for "package discovery" on setuptools docs.
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build editable did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

The solution is to explicitly define where to look for packages (the AFMReader directory) and at the same time exclude
tests/.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.81%. Comparing base (f282c31) to head (6d9710c).
Report is 196 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
+ Coverage   65.87%   71.81%   +5.94%     
==========================================
  Files           3        8       +5     
  Lines         337      589     +252     
==========================================
+ Hits          222      423     +201     
- Misses        115      166      +51     

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