FIX: enable importing pyotb after tensorflow has been imported

Closes #2 (closed)

This now works:

import tensorflow
import pyotb

Limitations: It is not possible to use any Tensorflow related applications if Tensorflow itself has been imported beforehand in the script:

import tensorflow
import pyotb

pyotb.BandMath(...)  # this is ok
pyotb.TensorflowModelServe(...)  # this will crash