Models: Fail if submodule not installed

[fleimgruber]

Expected behaviour
A git clone should work out of the box or error / warn if dependencies are missing.

Actual behaviour
Git clone is installed correctly in develop mode. Module can be imported, client can be instantiated, but has the attributes client.model_1, client.model_111, ... instead of the expected client.common, client.inverter, ...

Steps to reproduce

git clone https://github.com/sunspec/pysunspec
pip install -e pysunspec

and then try an example such as https://pysunspec.readthedocs.io/en/latest/pysunspec.html#interacting-with-a-sunspec-device.

Suggested fix
Error out or warn the user / developer that the model definitions could not be found.

Notes
The README states that the clone should have --recursive for the git submodules, but that may be overlooked in development, customized or automated installations. This is to warn / fail early in case of an inconsistent and non-functioning installation.

[fleimgruber]

I had a closer look and it seems that the branch

pysunspec/sunspec/core/device.py

Line 907 in 5242ea9

except:

is not followed because model_type_get(1) does not throw any exception.