Support ordinal categories for the R export

[esyriani]

Currently, the export to R feature only supports categories from the data extraction form that are continuous or nominal. We want to support also ordinal categories. In the data extraction form, an ordinal category is a list or dynamic list where the textual values have an order. For example, "maturity levels" (M1, M2, M3), "coverage" (10%, 50%, 75%), etc. They are strings that need to be assigned an integer order.
Steps:

• Add an optional "order" field in the "Reference categories" (form and database). This covers dynamic lists. For lists, the order of the values specified in the configuration file of the project will be the assigned order.
• Add the "Ordinal" option in the "R export configuration" page. A warning should appear saying to make sure all values have been ordered (with a link to the corresponding page of the reference category). If the order is not specified for at least one of the values of this category, then a default integer will be assigned according to the order in the database.
• In the generated R library file, ensure that they get the same descriptive and evolution statistics as continuous variables.
• For the comparative statistics, choose the appropriate test when comparing variables. Also test for independence above a threshold to determine the test.