Remove theano imports

[MichalChromcak]

Hey,

I was browsing through the codebase and found it very interesting, considering playing a bit more with it.

I saw then, that in some modules, there seems to be an unnecessary dependency on the theano.

timeseers/src/timeseers/constant.py

Line 6 in 50275a4

import theano.tensor as tt

No big issue, but more accessible if this would be removed while being quite a quick fix. What do you think?

[MBrouns]

This took way too long, but it's fixed in #32