Added code from deepcell repo and changed dependencies in pyproject.toml

[JLrumberger]

What is the purpose of this PR? We want to exclude deepcell from the Nimbus dependencies, because the library causes dependency conflicts with arc-analysis.

How did you implement your changes

The required code from deepcell has been copied over to this repository in the module deepcell with the following folder structure

Nimbus/
|-- src
|   |-- cell_classification
|   |   |-- nimbus.py
|   |   |-- ...
|   |-- deepcell
|   |   |-- application.py
|   |   |-- ...
|-- tests
|   |-- nimbus_test.py
|   |-- deepcell_XYZ_test.py
|-- pyproject.toml
|-- conftest.py
|-- setup.py
|-- README

After copying the code and excluding deepcell from dependencies in pyproject.toml, loading the model checkpoint into the deepcell application threw an exception which I fixed by changing how we construct the path to the checkpoint file.

Remaining issues None

[srivarra]

@JLrumberger Coveralls is good to go!