Support import of several external mapper packages

[korvinos]

At the moment the nansat supports the import of only one custom package of external user-developed nansat_mappers. However, it can be useful to allow the import of an arbitrary number of external mappers related to different top-level projects e.g.:

- project_1
| - nansat_mappers_project1
| - other_project_code 
...
- project_2
| - nansat_mappers_project2
| - other_ project_code 
....

To support this feature the _import_mappers method from the nansat module should be reworked.

[korvinos]

Hi @akorosov, it seems for me like a reasonable enhancement for the nansat and I guess I have a not too bad idea about how to implement it. Do you think it could be useful?