Replace ExecutionParameters in the public API

[alecandido]

I would expect the change to be pretty minimal: use kwargs instead of options in Platform.execute(sequences, sweepers=None, **kwargs) and do

options = ExecutionParameters(**kwargs)

right after. Also, remove ExecutionParameters from the public API and I guess some updates in the docs/README. If it's only that we could probably do in the short time we have, if it ends up being more complicated certainly no.

Whether it's sufficiently advantageous I am not sure. It should make Platform.execute slightly easier to use, particularly for users who are not interested in the whole content of ExecutionParameters (and only care about say nshots), but even this may be subjective.

Btw, I could also do that, if you agree.

Originally posted by @stavros11 in #1030 (comment)

We're attempting this for 0.2 - if not sufficiently stable, it will be moved directly to 0.3