Re-work boundary tests

[dstenger]

Several tests state that each boundary shall contain exactly one surface element. However, the code is checking if any boundary contains a surface element. Also, inheritance is not considered when testing abstract elements.

Following tests are affected:

• Abstract Test 15 - https://github.com/opengeospatial/ets-citygml30-part2/issues/6
• Abstract Test 37 - https://github.com/opengeospatial/ets-citygml30-part2/issues/17
• Abstract Test 22 - https://github.com/opengeospatial/ets-citygml30-part2/issues/8
• Abstract Test 19 - https://github.com/opengeospatial/ets-citygml30-part2/issues/9
• Abstract Test 29 - https://github.com/opengeospatial/ets-citygml30-part2/issues/12
• Abstract Test 24 - https://github.com/opengeospatial/ets-citygml30-part2/issues/10
• Abstract Test 35 - https://github.com/opengeospatial/ets-citygml30-part2/issues/16
• Abstract Test 39 - https://github.com/opengeospatial/ets-citygml30-part2/issues/18
• Abstract Test 43 - https://github.com/opengeospatial/ets-citygml30-part2/issues/20
• Abstract Test 17 - https://github.com/opengeospatial/ets-citygml30-part2/issues/7
• Abstract Test 9 - https://github.com/opengeospatial/ets-citygml30-part2/issues/4