Merging hills and hills should result in mountains

[antonhensen81]

Is your feature request related to a problem? Please describe. Currently, hills and hills cannot be merged.

Describe the solution you'd like Merging hills and hills should result in mountains

Describe alternatives you've considered None

Additional context There is a test in test_tile_type that test exactly this, uncomment the following line to make the test run and make it pass:

//assert(get_merge_type(tile_type::hills, tile_type::hills) == tile_type::mountains);