fix conda check for IPython<7.0.0 #41
Description
If the IPython %conda magic command fails, davos.implementations.ipython_common._check_conda_avail_helper() returns None and davos.core.core.check_conda() sets davos.config.conda_avail to False. The magic command is assumed to fail because the conda executable doesn't exist, however it will also fail with IPython<7.0.0 since it wasn't yet implemented. The _check_conda_avail_helper() function in davos.implementations.ipython_common should be moved to davos.implementations.ipython_post7, and an implementation that doesn't rely on the %conda magic should be added to davos.implementations.ipython_pre7.
Unfortunately, as far as I can tell, there's no reliable way to identify the conda executable from the kernel environment (i.e., rather than the notebook server environment) short of reimplementing the %conda magic command's logic, so that's probably what will need to be done.
For now, this is a low impact issue since conda-based installation, package versioning, and stdout parsing isn't supported yet anyway.