some tensorflow versions not available in conda

[FynnBe]

required tensorflow (version) is not available through conda...

Looking for: ['bioimageio.core', "tensorflow[version='>=1.12,<1.13']"]
  
  
  Encountered problems while solving:
    - package tensorflow-1.12.0-gpu_py27h956c076_0 requires tensorflow-base ==1.12.0 gpu_py27h8e0ae2d_0, but none of the providers can be installed
    

Originally posted by @FynnBe in #441 (comment)

[FynnBe]

I suggest we use pip to install any tensorflow 1 dependencies (given the tensorflow verison is only specified via tensorflow_version and not through a pip::requirements.txt or conda::environment.yaml