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Makes room for an eventual pybind11::embedded target.

Add a BUILD_INTERFACE and a pybind11::pybind11 alias for the interface
library to match the installed target.

Add new cmake tests for add_subdirectory and consolidates the
.cpp and .py files needed for the cmake build tests:

Before:
tests
|-- test_installed_module
|   |-- CMakeLists.txt
|   |-- main.cpp
|   \-- test.py
\-- test_installed_target
    |-- CMakeLists.txt
    |-- main.cpp
    \-- test.py

After:
tests
\-- test_cmake_build
    |-- installed_module/CMakeLists.txt
    |-- installed_target/CMakeLists.txt
    |-- subdirectory_module/CMakeLists.txt
    |-- subdirectory_target/CMakeLists.txt
    |-- main.cpp
    \-- test.py

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.

Add unit tests and documentation for the chrono cast.

This build makes sure everything still works without optional
dependencies (numpy/scipy/eigen) and also tests the automatic
discovery functions in CMake (Python version, C++ standard).

[skip appveyor]

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

Mark the pybind11 headers as private to the target.

Fixes #305

The function creates a pybind11 module using the specified source files.

This is needed in case an older pybind11 version was installed with pip