Compacts code for level 3 file generation

[martinwiesmann]

In this merge, I merged the SPICE_DATA method xcfit_block into the create_l3 method to avoid duplication of code. I also added a few new keywords to create_l3 and xcfit_block:

• approximated_slit: This keyword was already there for xcfit_block, added it also to create_l3. @tfredvik Should this be set by default, when creating l3 files?
• no_fitting: If set, cfit_block is not run, i.e. the fitting is not done. Useful for debugging.
• no_widget: If set, xcfit_block is not called after preparing the data, finding the peaks and possibly computing the fit. Useful for the creation of l3 file programmatically, without checking the results manually.

If both 'no_fitting' and 'no_widget' are set, then some of the data cubes are not being set (i.e. result, residual, include, const). So I have to make sure, that those are initialised, so that the FItS file can be saved.

Also updated, the wiki and the README.

[tfredvik]

approximated_slit should not be set as default. Not yet, anyway. If it turns out that the code for determining the slit height doesn't work that well and I'm not able to get it to work reliably, the we should consider setting approximated_slit as default.