Some refactorings needed for the GUI

1) Removed ```end_year``` argument hack. Now ```end_year``` is an attribute of the ```Catalogue``` set at creation time (and never modified)

2) Changed the signature of the occurrence methods to make them accept in input a ```Catalogue``` instance instead of a plain dictionary

3) Created the ```Grid``` input type to be used in the ```smoothing``` subpackage

4) Removed trailing white spaces (this accounts for most of the diff)

5) Fix a bug occurring when the smoothing kernel is called on a grid where the sizes are not divisible by the spacing

For a clean diff, https://github.com/GEMScienceTools/hmtk/pull/20 should land first.