add merged product

[pat-schmitt]

In this PR I added the merged product, consisting of the following:

• shape files of the intersects of individual glaciers
• shape files of merged outlines, and a csv file only containing the attributes
• json file to link individual glacier ids with merged glacier ids

Some thoughts/questions:

• should intersect lines be densified (e.g. define a maximum-line-length between points along a line using https://shapely.readthedocs.io/en/latest/reference/shapely.segmentize.html)
• currently, no minimum total length for intersects used
• attributes of merged product: currently I only added area, are there other attributes I should add?
• the naming of resulting files: I decided to use I for intersects, C for merged glacier complex, C-to-G for the conversion list

[pat-schmitt]

cenlon and cenlat I should definitely add to the complex product. But what about the attributes which require a DEM (zmin_m, slope_deg ...)?

[fmaussion]

Fantastic! Code for lon lat:

    rp = shp.representative_point()

    coordinates = np.array(list(rp.apply(xy_coord)))
    shp['CenLon'] = coordinates[:, 0]
    shp['CenLat'] = coordinates[:, 1]

[pat-schmitt]

Ok, just added cenlon and cenlat.