improved error message for Metric

[LonnekeScheffer]

previous error message was very generic (key F1 not defined), the new error message shows explicitly a list of valid options

[LonnekeScheffer]

• Added minor fix from PR 145: https://github.com/uio-bmi/immuneML/pull/145
• Added minor fix such that tests hopefully pass on github

[LonnekeScheffer]

All issues are resolved now. From my side, this branch can be merged in.