changing the user createmodel script interface to be more transparent

The createmodel script now provides the following options:

  --name=           name of the model

  --default-model=  name of the model from which the FlexibleSUSY
                    model file should be generated

  --sarah-model=    name of the underlying SARAH model

There are now the following use cases:

1) ./createmodel --name=MSSM

   Creates a FlexibleSUSY model named "MSSM", using the FlexibleSUSY
   model file from templates/MSSM/ and using the SARAH model "MSSM"
   (Start["MSSM"])

2) ./createmodel --name=MyMSSM --sarah-model=MSSM \
      --default-model=MSSM

   Creates a FlexibleSUSY model named "MyMSSM", using the FlexibleSUSY
   model file from templates/MSSM/ and using the SARAH model "MSSM"
   (Start["MSSM"])

3) ./createmodel --name=MyMSSM --sarah-model=MyMSSM \
      --default-model=MSSM

   Creates a FlexibleSUSY model named "MyMSSM", using the FlexibleSUSY
   model file from templates/MSSM/ and using the SARAH model "MyMSSM"
   (Start["MyMSSM"])