Improve usage of resource constraint in ListScheduler

There are two unhandled cases that need to be addressed:

1. If the provided amount of resources is lower than the theoretical lower bound the scheduler should not even try, just raise an exception to tell the user.

2. If the user does not provide any resources then -> try lower bound and then go up until success and raise a warning to tell the user what happened.

This will also work nicely when the user is able to specify a scheduling time. (as specified in #292 (closed))

added Scheduling label

changed title from Extend resource constraint in ListScheduler to Improve usage of resource constraint in ListScheduler

changed the description

assigned to @simbj106

1 has been done now

I think 2 can be "replaced" for now by adding a method in SFG that calculates the lower bound for a type, the user can then use this method when specifying resources.

mentioned in merge request !484 (merged)

closed with merge request !484 (merged)