add ability to set dynamic parameters

[sahilseth]

Currently, in flowr (like many other tools), all parameters are decided before execution.

There are a few use cases in which it might be useful, that step A; analyzes the data and sets a parameter for step B.

An example would be; we align reads and infer insert size. Then this is used as a parameter for tools like breakdancer (for looking at translocations).

http://bionics.it/posts/dynamic-workflow-scheduling