[BUG] Fix for import of MutableMappingBase
[DEV] New Configuration Package

Includes descriptors for configurations.
[DEV] Base uses new configuration property
Merge branch 'feature/config-pkg' into develop
Astronomy tools inside pyshell
Refactor of units to prevent returning copies of units in fancy properties.
Skeleton for the unit tests.
Fix to some metadata quirks in StructuredConfig
Readonly typed properties allow initialization.
Bound configuration action class
PyShell's PyYAML custom subclass maps number subclasses

This allows np.float and np.int to be correctly output by YAML
documents, without special tags.
Use the bound configuration action item.
Skeleton of a functional subclass for SCEngines
Merge branch 'feature/astron' into develop
Fixes for error classes and debug arguments.
Unit properties adjustments.