Create population workflow (stub) from template

[Yuri05]

First draft of a template for R for discussions: WorkflowInput.xlsx

• sheets starting with an underscore can be ignored for now (they come partly from the old matlab template and will be replaced)
• TODO: sheet with a study design (should have more or less 1:1 the structure of the current study design files)

some old (Matlab ) usage examples:

s. https://github.com/Open-Systems-Pharmacology/Reporting-Engine/tree/develop/advanced_examples For each example: workflow.m was automatically generated from the corresponding WorkflowInput.xlsx by using [prepareWorkflow](https://github.com/Open-Systems-Pharmacology/Reporting-Engine/blob/develop/src/code/templates/prepareWorkflow.m), e.g. * [01_MeanModel_Raltegravir/WorkflowInput.xlsx](https://github.com/Open-Systems-Pharmacology/Reporting-Engine/blob/develop/advanced_examples/01_MeanModel_Raltegravir/WorkflowInput.xlsx) ==> [01_MeanModel_Raltegravir/workflow.m](https://github.com/Open-Systems-Pharmacology/Reporting-Engine/blob/develop/advanced_examples/01_MeanModel_Raltegravir/workflow.m) * [05_Population_RatioComparison_Raltegavir/Workflow_input.xlsx](https://github.com/Open-Systems-Pharmacology/Reporting-Engine/blob/develop/advanced_examples/05_Population_RatioComparison_Raltegavir/Workflow_input.xlsx) ==> [05_Population_RatioComparison_Raltegavir/workflow.m](https://github.com/Open-Systems-Pharmacology/Reporting-Engine/blob/develop/advanced_examples/05_Population_RatioComparison_Raltegavir/workflow.m)

[Yuri05]

@pchelle @abdullahhamadeh @msevestre @sfrechen @TWendl First draft of the new template attached in the issue description