Connections to cell type and abc databases

[srirampc]

Addresses issues #17 and #18 :

• Added Query classes to query cell type data from Allen Cell Feature databases using AllenSDK
• Added Query classes to query neuron type rations from Allen Brain Cell Atlas using ABC Cache
• Implement a workflow that (a) downloads data , (b) applies transformations on the data, (c) Build a model driven by the data and (d) save the model as a SONATA file
• Functionality to configure the workflow using json configuration files:
• Save model descriptions and the configurations.