v0.1 spice data can be looked at

[martinwiesmann]

v0.1 spice_raster_browser and the spice_data object can now handle SPICE FITS files, tested with the two test files. Dumbbells are still completely ignored.

• all methods of the raster_browser were adapted to spice_data object methods
• added and edited some comments
• fixed some bugs:
  • FITS keywords with dashes are translated to '_D$' in a IDL structure, fix handles that now
  • fixed an older bug, where program crashed when calculating spectrum at edges of image
• exchanged the test files with l1 test files

[martinwiesmann]

I would squash this merge pull request as well, as the other two before. Then, I will create a merge pull request for develop -> master, since I think this is a first working alpha version.