Refine `ExpressionInput` to `CheckExpressionInput` and `EvaluateExpressionInput`

[bjornbugge]

The class ExpressionInput on the server side has been split up into the two classes CheckExpressionInput and EvaluateExpressionInput for, respectively, the /check endpoints and the /contract/{id}/report endpoints. They both have a field called csl with a string containing CSL code, but the EvaluateExpressionInput has an optional field values that is a list of Value objects.

The various REST methods for check and report should be changed accordingly.

[bjornbugge]

Closed by #4