Error at runtime if zfpy isn't present

[dstansby]

This is a draft proof of concept for #655, to show what defining the codec classes, but erroring at runtime (with a helpful error message) would look like. Currently just implemented for ZFPY.

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.67%. Comparing base (9bdbaf0) to head (57cddff).

Files with missing lines	Patch %	Lines
numcodecs/zfpy.py	95.74%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (9bdbaf0) and HEAD (57cddff). Click for more details.

HEAD has 9 uploads less than BASE

Flag	BASE (9bdbaf0)	HEAD (57cddff)
	12	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #666       +/-   ##
===========================================
- Coverage   99.92%   89.67%   -10.25%     
===========================================
  Files          62       62               
  Lines        2732     2732               
===========================================
- Hits         2730     2450      -280     
- Misses          2      282      +280     

Files with missing lines	Coverage Δ
numcodecs/__init__.py	100.00% <100.00%> (ø)
numcodecs/zfpy.py	95.91% <95.74%> (-4.09%)	⬇️

... and 2 files with indirect coverage changes

[dstansby]

This diddn't work - see #655 (comment) for context.