Implement deserialization from .ga files

[loichuder]

For #1

A Galaxy workflow is made of "steps". Among these steps, there are tools which perform some operations (CLI-based). These are basically Ewoks tasks with task_type = script so no problem for this.

However, Galaxy has a data_input concept which is a step that loads data from an arbitrary source. There is no equivalent ,that I know of, in Ewoks. I decided to translate this in a Ewoks task LoadDataInput that has to be implemented.

Also, data_input step have inputs that are stored in a ambiguous way. I could not really find the equivalent in Ewoks.

To add confusion, some inputs seem to be also in a field tool_state?

And the format reference didn't really help me to figure things out.

[loichuder]

After discussion, it is probably easier to work on the serialization first (Ewoks to Galaxy). I'll draft this for now.