Can't run the PI with only coupled/group PI parameters

[dlefaudeux]

When trying to run a parameter identification I ran into a error:
"Error in .validateSimulationIds(ids, parameters, outputMappings) : Mismatch or missing ID detected: [...]. Ensure each Simulation ID matches with corresponding PIParameter and OutputMapping IDs."

It seems to be happening when only grouped PI parameters are given.

How to reproduce:
Using the example in the user-guide, trying to run only on the lipophilicity doesn't work:

works (example in the user-guide):

piTask <- ParameterIdentification$new(
  simulations = list(sim_250mg, sim_500mg),
  parameters = list(piParameterLipo, piParameterCl_250mg, piParameterCl_500mg),
  outputMappings = list(outputMapping_250mg, outputMapping_500mg),
  configuration = piConfiguration
)

doesn' t work:

piTask <- ParameterIdentification$new(
  simulations = list(sim_250mg, sim_500mg),
  parameters = piParameterLipo,
  outputMappings = list(outputMapping_250mg, outputMapping_500mg),
  configuration = piConfiguration
)

@rengelke