Add tests for floating point transforms in FixedScaleOffset

[dstansby]

Some tests for floating point parameters in FixedScaleOffset, to go with #356.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (6fb781b) to head (cb2a8ae).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #566   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          59       59           
  Lines        2407     2408    +1     
=======================================
+ Hits         2405     2406    +1     
  Misses          2        2           

Files with missing lines	Coverage Δ
numcodecs/tests/test_fixedscaleoffset.py	100.00% <100.00%> (ø)