Add PR error plotting and more docs

bob/ip/binseg/engine/inferencer.py

bob/ip/binseg/script/binseg.py

bob/ip/binseg/utils/evaluate.py

bob/ip/binseg/utils/metric.py

bob/ip/binseg/utils/pdfcreator.py

bob/ip/binseg/utils/plot.py

bob/ip/binseg/utils/rsttable.py

doc/benchmarkresults.rst

doc/covdresults.rst

doc/img/pr_CHASEDB1.png

doc/img/pr_DRIVE.png

doc/img/pr_HRF.png

doc/img/pr_IOSTARVESSEL.png

doc/img/pr_STARE.png

doc/index.rst

doc/plotting.rst