Add utility functions

[PaulKalho]

Corresponding to #5 we currently support 3 types of inputs.

These are the following:

env
db_table
file

For each we need to add utility functions to the sdk to access the data using the information provided/defined within the cbc.yaml

E.g.

env => read/parse env file, throw if not set
db_table => connect to db, read, write, ...
file => connect to s3, read, write, ...

Important: As we are using Apache Spark to process our data we should definetly use their methods of handling data. It is important to take a closer look at these. (e.g. RDDs, how can we make them accessible to the sdks user very easily)

We could also add utility functions for testing input/outputs (connections etc.) before the actual workload starts.