dependencies in docs/model/makefile to building images (#149)

Build images.dir when building report.pdf
Add outputs to .gitignore

docs/.gitignore

+model/appendix.fdb_latexmk
+model/appendix.fls
+model/formulation.fdb_latexmk
+model/formulation.fls
+model/report.fdb_latexmk
+model/report.fls
+model/symbols.fdb_latexmk
+model/symbols.fls
+passive_cable/cable_computation.bbl
+passive_cable/cable_computation.bcf
+passive_cable/cable_computation.fdb_latexmk
+passive_cable/cable_computation.fls
+passive_cable/cable_computation.run.xml
+passive_cable/cable_computation.xdv

docs/model/makefile

-report.pdf : *.tex
+report.pdf : *.tex images.dir bib
 	pdflatex report.tex
 
+images.dir:
+	make -C images
+
 force : report.pdf
 	pdflatex report.tex