added fill_value
Merge pull request #8 from wkerzendorf/fitter/fix_nan

added fill_value
Add Normalise to the plugins, plus test case (plus PEP8)
Add NormalizeParts plugin, as well as test for it

Note that this includes a hack to slice Spectrum1D objects
PEP8 on fitting.py
added an InstrumentConvolve that uses a resolution and interpolates on a logarithmic grid to do wavelength independent convolution