Otherwise, this header is installed if BUILD_PYTHON_INTERFACE, and not
if INSTALL_PYTHON_INTERFACE_ONLY.

But if we want to be able to have binary packages for:
- the main curve package
- its bindings for python 2
- its bindings for python 3

this header will be installed in the 2 last cases, which will conflict.

In short: if we set INSTALL_PYTHON_INTERFACE_ONLY, there should be only
stuff installed in PYTHON_SITELIB.

Previously, we only had curves.so, which could be installed in
PYTHON_SITELIB.

Now, we also want to ship plot.py, so we need the following final
structure:
${PYTHON_SITELIB}/
└── curves/
    ├── __init__.py
    ├── curves.so
    └── plot.py

Then we can keep the same structure in the build dir, to be able to
run tests before any installation:

build/
└── python/
    └── curves/
        ├── __init__.pyc
        ├── curves.so
        └── plot.pyc

And for this, we can once again keep the same structure in the source
dir:

python/
└── curves/
    ├── __init__.py
    ├── curves_python.cpp
    └── plot.py

Edit file name spline_python.cpp to curve_python.cpp, in test.py edit TestSpline and test_spline to TestCurve and test_curve, edit all makefiles, make test 100% OK