Doc parameters mandatory

This merge request responds to several requests of issue #234 (closed):

• add a dictionary to link a builder name to a mandatory or optional parameters
• when the chain starts, iota2 automatically check all mandatory parameters

RST generation has been modified too:

• a new column for mandatory was added with boolean value: True : the parameter is mandatory for the builder
• the parameter name is duplicated at the end of the table to ease the reading of long table
• each parameter have now a unique explicit link formatted by the builder name, the section and the parameter name: using

 ref:`i2_classification.chain.data_field` or i2_classification.chain.data_field_ 

creates a link in documentation to access the corresponding table.