'Evaluates and sets the specified option value in
environment `loc`. Many options need `N` to be defined in
`loc`, some need `popsize`.
Details
Keys that contain \'filename\' are not evaluated.
For `loc` is None, the self-dict is used as environment
:See: `evalall()`, `__call__`'

def eval(self, key, default=None, loc=None):

    self[key] = self(key, default, loc)
    return self[key]
