Feature: strong type conversion [1/2]

[holke]

Closes #1664

Describe your changes here:

Added implicit type conversion to base type to our strong types.

Marked [1/2] since it has to be merged before #1656 which uses this feature.

Why?
In some scenarios, in particular when we want to print out values for our strong types, we need to cast them back into their base type.
For example when strong types are used in GoogleTest assertion, GoogleTest requires the "<<" operator to work.
It happened to me when switching to a strong type for geometry hash usage (see #1656) and a part of the code was doing
t8_debug_f ("Hash %lu not found\n", hash); which did not compile since "hash" as a strong type was not convertible to "long unsigned".

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