add possibility to verify cumulative tiles

[CyberGuerro]

add to be able to verify cumulative tiles for example this:

Tiles/zOrtho4XP_Hawaii
             |
             |--- Earth nav data
             |      |--- +10-160
             |             |--- +18-156.dsf
             |             |--- +19-155.dsf
             |             |--- +19-156.dsf
             |             |--- +19-157.dsf
             |--- terrain
             |       |--- ter files
             |--- textures
             |       |--- texture files
             |--- Ortho4XP_+18-156.cfg
             |--- Ortho4XP_+19-155.cfg
             |--- Ortho4XP_+19-156.cfg
             |--- Ortho4XP_+19-157.cfg