Adds CMake library and install target

[timothyqiu]

This PR

• adds an interface library target Clara, aliased Clara::Clara

• corresponding install entries are also added

  clara-install-prefix/
  |-- include
  |   `-- clara.hpp
  `-- lib
      `-- cmake
          `-- Clara
              |-- ClaraConfig.cmake
              `-- ClaraConfigVersion.cmake
  

  so find_package(Clara REQUIRED) can properly import Clara::Clara from the installed package.

• include directories needed for ClaraTest targets are now set using target_include_directories instead of changing global settings

This implements the main part of #44

[timothyqiu]

😞 One CI job failed because it can't pull code from GitHub, I don't know how to trigger that job again...

[codecov]

Codecov Report

Merging #76 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   94.85%   94.85%           
=======================================
  Files           2        2           
  Lines         680      680           
=======================================
  Hits          645      645           
  Misses         35       35

Impacted Files	Coverage Δ
include/clara.hpp	93.3% <0%> (ø)	⬆️

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