Add log-uniform period distributions
Overhead model: add support for loading custom overheads from files

Rationale: callers may want system-specific overheads to be loaded
when instantiating an overhead model from a file.
Add support for loading overhead files with tasks-per-core task counts

When loading an overhead model in which the number of tasks is given
per core, it is convenient to scale up the task count while loading
the file. OA