reformat Energy2D *.e2d xml files

Makes the structure in the files easier to read manually.

$ find imports/energy2d/content/ -name '*.e2d' | xargs -I % xmllint --format % --output %
$ find imports/energy2d/models-xml/ -name '*.e2d' | xargs -I % xmllint --format % --output %

Also manually remove \n from empty <sensor> elements to make
it easier to determine when the sensor element is empty.

> Hash.from_xml("<sensor>\n</sensor>")
 => {"sensor"=>"\n"}

> Hash.from_xml("<sensor></sensor>")
 => {"sensor"=>nil}