variables.csv: allow more variable types

[JuanLiOHRI]

Following Issues #18: variables.csv: add column "typeStart", I'd like to suggest adding more allowed variables types than categorical or continuous, especially in "typeStart":

1. For continuous: do we need to separate "integer" from "double/float" (with allowed decimal places)?
2. For categorical: maybe we need to separate "categorical" from "ordinal" variables. This may also apply to "typeEnd" since they should be treated differently in the following data transformation and analysis.
3. Text input, like comments of "Other" or other annotations, needs some extra and often user specific transformations.
4. Date and time: they are often used to derive other continuous variables, like age or duration of disease. But it will require some special packages and functions to deal with this data type.