Make automatic module import failsafe

The current version of the codebase can not be used in a headless setup, i.e. without access to tikinter for displaying UIs, since __init__.py automatically imports all modules, including the ones used for plotting.

I would propose to make the automatic import failsave by catching and displaying import errors.

Steps to repro

Clone repository; install dependencies specified in setup.py and execute from root, on a setup without GUI access (i.e., without an X11 server running)

python3 -c "import skinematics"

Expected behavior

The parts not requiring a GUI should be imported without any errors.

Actual behavior

We get this error in a headless setup

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/steffen/scikit-kinematics/skinematics/__init__.py", line 29, in <module>
    importlib.import_module('.'+_m, package='skinematics')
  File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/home/steffen/scikit-kinematics/skinematics/imus.py", line 40, in <module>
    import quat, vector, misc, rotmat
  File "/home/steffen/scikit-kinematics/skinematics/misc.py", line 25, in <module>
    import tkinter 
ModuleNotFoundError: No module named 'tkinter'

It is possible to install the required packages e.g. in Ubuntu; then the error fails on the tikinter import, e.g. with an OpenGL error message.

Proposed Fix

Partition the modules into required and optional modules. Catch the ModuleImportError for all optional modules. This makes it possible to run the same code base on both headless and X11 based systems.

Applying the fix yields this behavior:

python3 -c "import skinematics"
Failed to import optional module imus. Install optional dependencies
Failed to import optional module misc. Install optional dependencies
Failed to import optional module view. Install optional dependencies

I am happy for suggestions on also adapting setup.py as part of this PR.

thomas-haslwanter / scikit-kinematics