Refactor geoflow action fun args

[eblondel]

Relates to #261 This is the current status regarding the way actions can be interfaced:

• global actions (embedded or custom - provided by user) have to be defined with a namedfunction interface like:

my_fun <- function(entity, config, options){}

where my_fun should be the action id.

• local actions: for which no function has to be provided

This is highly criticizable and not really convenient for the user, for multiple reasons detailed below:

• Not the same logic is applied for global vs. local actions. Proposal: handle a function for all actions
• For global actions, giving a function name is useless, in particular for custom actions where we constrain the user to name the function with the id defined for the action.
• The options are not clear, especially for testing local actions, these are the action options. This 'argument' should be removed from the function interface. The action definition should be made available. From there, options will be easily grab using action$options, or using util to develop in #261 which will then avoid re-writing action options default values in the action function, but inherit them from the action definition;
• This will open the door to test implementation of #125

Hence, any action (global embedded, global custom, local) will have to be designed within a function interface as follows:

function(action, entity, config){

   opts <- action$options
   ....
}

In addition, for global embedded actions, an attempt will be made to create a separate folder 'actions' under the 'inst' folder

@abennici @juldebar @wheintz if you have any questions, feel free. I will anyway assist you in changes where needed

[bastienird]

Are the global custom actions not concerned by this update ? As it still mandatory to have an "options" argument in the function ? (Line 517 of geoflow::initWorkflow https://github.com/eblondel/geoflow/blob/master/R/initWorkflow.R)

[eblondel]

All actions should be concerned by this code refactoring/harmonization