fix obpg l2 mapper
#79 replace multiple download function with one; replace multiple mapper_test functions with one
#79 use env variable MAPPER_TEST_DATA_DIR
#79 add obpg_l2 test data; allow to use env.var MAPPER_TEST_DATA_DIR
#79 make opening of each file a separate test
#79 rename test calss for DataForTestingMappers; add license information
Merge branch 'issue79-test-obpg-l2' into develop