Avoid costly input generation

[andsor]

We need to take special care for generating costly input because basically, the backends re-calculates all the inputs every time a new task is called by the jug scheduler

so for now, I propose to delegate and encapsule costly input generation to the simulation function

[andsor]

Update after discussion with @debsankha

In contrast to the above, we shall treat each iteration of the input generator as a computational task in its own right (e.g. for costly generation of input).

Questions:

• What should be the default behaviour -- own task or not?
• How does the user specify the other behaviour -- decorating the generator, supplying another argument to the experiment initializer or the caller?

References:

• https://jug.readthedocs.org/en/latest/tasks.html#what-should-be-a-task