Feature/MultiParameterWithSetpoints

[thibaudruelle]

Changes proposed in this pull request:

• Add MultiParameterWithSetpoints, which is mostly the same as ParameterWithSetpoints except the get function returns a Sequence of Arrays
• Update expand_setpoints(), Dataset.add_results() and Measurement.add_parameter() to accept MultiParameterWithSetpoints

TODO:

• [ ] add tests ?
• [ ] solve possible integration issues

@Dominik-Vogel