Improves handling and assignment of default units
When creating a material without specifying units, no units are
currently assigned. This commit will assign the default units when none are
given, or throw an error when incompatible units are given. In the
latter case, the units are set to the property defaults.
This commit also incidentally fixed an issue when saving the material that
resulted in accessing an uninitialized pointer.
The default display type of quantity objects is fixed point resulting
in insufficient accuracy when changing unit systems, or when the values
are small. This fix changes the default format from 'Fixed' to the more
apt 'Default' format. This allows the displayed values to scale as
appropriate.
Fixes#18149
Corrects an issue in the API where a new material may not have a UUID.
Corrected the test case to reflect the changes and better document the
process.
Added a test case for material filters.
