Warn the user when shape or chunks contains float values

[faymanns]

When shape or chunks contains float values, the digits after the decimal point are trimmed.
This pull request generates a UserWarning when one or multiple values in shape or chunks are non-integer.
The problem is described in more detail in #2530 and is already fixed in v3 by PR #2535.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (2ab280a) to head (1ae98fd).
Report is 1 commits behind head on support/v2.

Additional details and impacted files

@@             Coverage Diff             @@
##           support/v2    #2579   +/-   ##
===========================================
  Coverage       99.95%   99.95%           
===========================================
  Files              38       38           
  Lines           14666    14672    +6     
===========================================
+ Hits            14660    14666    +6     
  Misses              6        6           

Files with missing lines	Coverage Δ
zarr/tests/test_creation.py	100.00% <100.00%> (ø)
zarr/tests/test_util.py	100.00% <100.00%> (ø)
zarr/util.py	100.00% <100.00%> (ø)

[dstansby]

Thanks a lot, this looks good!