Clarify which variables can be set in Continuous Time Mode

[t-sommer]

According to the table in 3.2.1. State: Continuous-Time Mode it is allowed to set continuous states in continuous time mode via fmi3SetFloat:

Set continuous-time states xc(t): fmi3SetFloat64, fmi3SetContinuousStates

however the description for fmi3Set{VariableType} contradicts this

Function fmi3Set{VariableType}
Only for variables with causality = input and variability = continuous.

see also modelica/Reference-FMUs#508

[t-sommer]

Proposal by @KarlWernersson: allow setVariable{VariableType} for continuous time states as well (regardless of attributes) as defined by the <ContinuousStateDerivative> in the Model Structure.