Complete redesign with simplified meta classes.

API not compatible to previous version!
Required Python version set to 3.7.
Updated required version of 'decimalfp' to 0.11.4.
Added 'pytest' as required for tests.
Ported tests to 'pytest'.
Added module defining SI prefixes.
Reformatted code and fixed names according to PEP 8.
Added and revised test cases and type hints.