add a bit of doc

doc/developer_manual.md

+This is a stub for a future documentation for developers.
+
+
+## Graph initialization
+
+The main steps for any graph initialization messily use functions across several modules, so it can be useful to list them here together:
+
+ 1. [`main.js`] initial choice of data source from URL params or settings
+ 2. [`sigma.parseCustom.js`] prepares the data
+     - read a source via ajax GET
+     - *"scan"*: loop once to list present node categories
+     - *"dictify"*: loop again to copy all nodes/edges information
+     - prepares TW.Relations: edges sorted by type (term-term, term-doc...)
+     - prepares TW.Clusters: bins and facet index (node attr vals => nodes)
+ 3. [`main.js`] precomputes display properties (grey color, etc.)
+ 4. [`sigmaUtils`] the function `FillGraph()` was a central point for filtering and preparing properties but now with 2 and 3 it just copies the nodes and edges to a new structure that groups them together
+ 5. [`main.js`] Finally all sigma settings (user + defaults) are merged and we initialize the sigma instance (`new sigma` etc.)
+
+
+#### About source data
+ - doc/sem typing: follows the node property "`category`"
+   - if the category name is "document"  => catSoc (type 0)
+   - if the category name contains the str "term"  => catSem (type 1)
+ - coloration:     "`age`" "`growth_rate`" + any attribute of type float or int
+ - clustering:     "`cluster_index`" ou nom figurant dans `TW.nodeClusAtt`
+
+## User interaction mecanisms
+
+#### Bindings
+  - selection is bound to sigma click events (cf. [Sigma JS Events API](https://github.com/jacomyal/sigma.js/wiki/Events-API))
+  - most event listeners are created in Tinaweb.initListeners(), called by main.js
+
+
+#### Selection mecanisms
+Rich selection possibilities is one of the main features added by TinaWebJS to the basic sigma capabilities.
+
+They are handled in Tinaweb.MultipleSelection2.
+
+###### module flags and events
+  - for a new selection:
+    - `TW.selectionActive` is set to `true` when there is a selection
+    - `tw:gotNodeSet` event is sent
+  - when deselecting
+    - `tw:eraseNodeSet` event is sent
+    - the function `cancelSelection()` is called
+
+###### node flags
+For any node `n` the relevant flags at selection are:
+  - `n.active` iff node is selected
+  - `n.customAttrs.highlight` if  node is a neighbor of a selected node