[Classification layer accuracy and area report] add support for non-matching category names

[janzandr]

Currently, observed and predicted category names must match.

E.g. use this setup to reproduce the problem:

It is proposed to soften this restriction a bit, and to also allow category matching by order, if the number of categies is equal.

[janzandr]

@jakimowb any remarks from your side?

[jakimowb]

It need to be cristal clear which class matching. By now it seems to be on class names only.

Optimally the processing algorithm should support the following options:

1. match on class names, e.g. if there is a raster and a reference shapefile with class names
2. match on class numbers
3. match on class order

The final report needs to show this as well, i.e. which used class names + numbers were compared.

[janzandr]

Thanks @jakimowb, I will add a description to the docs and add the matching to the report.

[janzandr]

FYI @jakimowb, done:

1. Class matching is shown in a table:
   

2. For classes that can't be matched by name, a warning is logged:
   

3. Improved algo doc: