Add data type specification in JSON primitives and conversion functions

[csala]

Define a standard data type for MLBlocks (pandas or numpy), and provide a way to specify whether a primitive works with this data type or needs a data conversion before or after it is used.

Also, implement conversion functions between the most common data types.

During runtime, the system should use the helper functions to convert the data types back and forth when using primitives that do not work with the standard type.
Also, the system should be smart enough to skip unnecessary conversions when passing data between blocks that support the same data type.

[csala]

Part of this will be covered in #43