Tuple variables

This MR extends the use of lists to tuples (fixed-size lists).

Description

• The default behavior is that miniflask (as for lists) assumes that all elements have the same type.
• It then checks for the first argument type of the tuple and uses its length to specify the number of arguments needed in case the CLI overwrites the variable.
• Using the wrong number of arguments exits the code with an error

Things done in this MR

• enabled tuples

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created

Example Usage

def register(mf):
    mf.register_defaults({'tuple_variable', (42,43,44))