More user friendly error message for import sinn

[alcrene]

In sinn.common, we should detect if a types have been loaded and frozen. A natural thing for a new user to do is type

import sinn

at the top of a session, and at present this throws the error

ImportError: cannot import name 'FloatX' from '<unknown module name>' (unknown location)

which is a very poor first impression. Instead we should detect this, provide a summary of the correct import sequence,

from mackelab_toolbox.cgshim import shim, typing
shim.load('theano')
typing.freeze_types()
import sinn

and refer to the a new readme section which explains in more detail how we deal with dynamic types.