Don't force users to install Cate Core

[forman]

Expected behavior

Cate Desktop shall not force users to install Cate Core anymore:

• If Cate Desktop cannot find any Cate Core, it shall prompt to install a new one (and inform what it is about). If the users doesn't want this, Cate Desktop will exit. Otherwise the user has the option to define the Cate Core installation path or use the default (e.g. ~/.cate/python).
• If Cate Desktop finds an existing environment which is not up-to-date, it shall prompt the user to update it. If the users doesn't want this, Cate Desktop will exit.

Installing and updating Cate Core from Cate Desktop avoids the need to have a dedicated Cate Core installer. We could simply install a Miniconda and then update it with the cate-core package from the ccitools channel. (May be deploy to Anaconda/conda-forge channel, but this is another issue.)

Actual behavior

We force users to install Cate Core in a previous step. This forces us to provide Cate Core installers (which is an extra effort).

Steps to reproduce the problem

Install Cate Desktop.

Specifications

Cate Desktop 1.0