Optionally Incorporate Units using unyt package

[umesh-timalsina]

It might be nice to have a nice little addon where in certain attributes of an Element will be tagged with Units if unyt package is available as a dependency. @rsdefever I will be happy to put this together if you agree to it.

[rsdefever]

In general I'm open to this idea if it is optional.

One question: will it make the behavior less well defined? Since sometimes quantities will contain units and other times they will not?