Add blender test function to check install blender+navipy

doc/source/tutorials/database.rst

+How to generate a database using blender
+----------------------------------------
+.. literalinclude:: examples/blenddemo_beesampling.py
+   :lines: 7, 8
+
+First we configure the rendering module
+
+.. literalinclude:: examples/blenddemo_beesampling.py
+   :lines: 11, 12
+
+With the toolbox at disposition we just need to configure the \
+BeeSampling to render images on a regular 3D grid.
+
+.. literalinclude:: examples/blenddemo_beesampling.py
+   :lines: 13,16-23
+
+If we want to use the distance to objects, we need to tell the \
+BeeSampling what is the maximum distance to objects in the environment.\
+ Otherwise the distance can go until infinity, and since the image are \
+compressed in the database, all distances to object will be equal to \
+zero:
+
+.. literalinclude:: examples/blenddemo_beesampling.py
+   :lines: 27-28
+
+Finally we can generate the database.
+
+.. literalinclude:: examples/blenddemo_beesampling.py
+   :dedent: 4
+   :lines: 31-32
+
+(:download:`Source code <examples/blenddemo_beesampling.py>`)
+(:download:`Blender world <../../../navipy/resources/forest_world.blend>`)

navipy/sensors/blendnavipy_test.py

+""" A small script to test if navipy can run under blender"""
+try:
+    import navipy  # noqa F401
+except ModuleNotFoundError:  # noqa F821
+    raise NameError(
+        'navipy could not be imported within blender scripting tool. ' +
+        'This script should be run with the command blendnavipy.' +
+        'If you did, you need to check that either the virtual' +
+        ' environment can be activated under blender, or that' +
+        ' the path can be reached by blender')
+
+print('Blender has been successfully imported')