Add models representing binding constrains (#25)

* Correct error in stock model

* Add model constraining total imports

* Add nuclear modulation model

* Add model with minimum electric vehicle load

* Add hvdc model

* Delete models because more efficient models are available with ports

* Small corrections on names and temporal sums

* Change model to take into account total import on a node

* Small corrections

---------

Co-authored-by: Juliette-Gerbaux <juliette.gerbaux_externe@rte-france.com>

models-design/model_library/stock_final_level.drawio

@@ -34,7 +34,7 @@
                 <mxCell id="48" value="r(h+1) = r(h) + u_in(h) - u_out(h)" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;spacingLeft=4;spacingRight=4;overflow=hidden;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;rotatable=0;whiteSpace=wrap;html=1;" parent="39" vertex="1">
                     <mxGeometry y="270" width="270" height="30" as="geometry"/>
                 </mxCell>
-                <mxCell id="49" value="flow_s, flow_c, flow_out : Flow" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;spacingLeft=4;spacingRight=4;overflow=hidden;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;rotatable=0;whiteSpace=wrap;html=1;" parent="39" vertex="1">
+                <mxCell id="49" value="flow_s, flow_in, flow_out : Flow" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;spacingLeft=4;spacingRight=4;overflow=hidden;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;rotatable=0;whiteSpace=wrap;html=1;" parent="39" vertex="1">
                     <mxGeometry y="300" width="270" height="30" as="geometry"/>
                 </mxCell>
                 <mxCell id="50" value="flow_s.f = u_out - u_in" style="text;strokeColor=none;fillColor=none;align=left;verticalAlign=middle;spacingLeft=4;spacingRight=4;overflow=hidden;points=[[0,0.5],[1,0.5]];portConstraint=eastwest;rotatable=0;whiteSpace=wrap;html=1;" parent="39" vertex="1">