Bug-Fixes in IO

Added some more documentations, added io.rst.
Converted scale to floats everywhere it is set by default.
Removed superfluous enumeration in reader/writer in io_field.
Fixed vector default and name saving in ovf.py.
Fixed vector default and necessity of dim in tec.py.
Fixed erronous dim -> scale assignment in vtk.py.
Fixed interp_to_regular_grid not using the multidim. scale correctly.