Add support for header-only libraries with tribits_add_library()

[bartlettroscoe]

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Description

With the move to modern CMake, it should now be easy to support header-only libraries with tribits_add_library(). The idea would be to provide a new keyword HEADERONLY that would allow the SOURCES <src0> <src1> ... argument to be missing. Then an interface-only CMake target would get created.

The hardest parts of adding this new feature is adding a test case for all of the supported use cases and adding documentation.

Motivating user is the STK package in Trilinos.