Change manipulate to act on Operator

[felixhekhorn]

Closes #203

There is no need for additional iterators as EKO is already providing all iterators needed. The only necessary change is to let manipulate act on Operator instead of an whole EKO.

I guess, we consider this as a part of our public API, so this will trigger a new version. This might also have an effect on evolven3fit (cc @scarlehoff )

As a nice side effect, we also remove a bug discovered in #291 : when activating plot_operators in ekomark we were crashing, since there was a manipulate and this confused eko afterwards

[felixhekhorn]

Since this is already breaking, I decided to put another breaking change on top: 3e2d12a drops the Bases class, i.e. metadata does not know about different bases (x,pid) in input or output any longer, which is consistent with the main change introduced in this PR

[felixhekhorn]

Since this is already breaking, I decided to put another breaking change on top: 3e2d12a drops the Bases class, i.e. metadata does not know about different bases (x,pid) in input or output any longer, which is consistent with the main change introduced in this PR

this addresses the first item of #205