Importing csrank causes error without optional theano installed

[kiudee]

Simply running

import csrank

after installing csrank from pip without any optional dependencies, results in the following error:

csrank.util.MissingExtraError: Could not import the optional dependency theano. Please install it or specify the "probabilistic" extra when installing this package.

This should definitely not happen.

Version: 1.2.0

[timokau]

Looks like this has been the case since optional dependencies were introduced. The problem is that we re-export everything at the top level, so import csrank will actually evaluate all the code. This will be solved by https://github.com/kiudee/cs-ranking/pull/115, though we probably want a faster intermediate solution.

[timokau]

I see two options:

1. make the optional imports more lazy, i.e. only import at usage and not at module import
2. make the optional imports mandatory again

(2) is probably best for a hotfix 1.2.1.