SectorCache::ClearCache to completely clear cache.

--HG--
branch : system_exploration
extra : rebase_source : e62f358abf1f036eaf59c2472a82b78a4c7f1771
Do not assign factions to sector when called from Faction code.

--HG--
branch : system_exploration
extra : rebase_source : 4939f2005199b520aa9608a9e310b4968601e5f6
extra : source : 7b16a1f91a1ff0f89f8d1eba7037859ad763f277
Avoid instantiating Sector directly for calculating distances.

--HG--
branch : system_exploration
extra : rebase_source : 5637ee7f663481949300bf9e3d3e97c76fa9ab13
extra : source : cb8185369689b4f0462c1951357c2f94962590a1
Make sure there is only ever one Sector object for each sector.

--HG--
branch : system_exploration
extra : rebase_source : 6162e388a5387c7bcf7a50c483cbe83fdeb291d4
extra : source : 39caeda0a83e8a553511f763155e99929eaee844
Move exploration state to Sector::System

--HG--
branch : system_exploration
extra : rebase_source : 6363ed117bf1090ad9350f044240c2978f2a97ba
extra : source : b6ea48d1049aedbec7be88d01cd29978a692d5ab
Allow unexplored systems to become explored in game.

--HG--
branch : system_exploration
extra : rebase_source : 73d1c7e2d95e856526010013e071e412827e9e9d
extra : source : 6970b9c5411726e7cc2a4f21dd5dc85149e3d0f6
Persist explored state of a system.

--HG--
branch : system_exploration
extra : rebase_source : 2630e324d1f3719a5ee9ea9f03186d45ef39a32a
extra : source : 9d52453f1492896e4519dcc4cfb0e557827802e4
LUA support for explored systems

--HG--
branch : system_exploration
extra : rebase_source : f4bbbacd4a392cb25e22fc4961fd51948e2e177a
extra : source : 4dcdeff0cd30fabd3daba426a65a1336790d35bf
Automatically explore system when entering it.

--HG--
branch : system_exploration
extra : rebase_source : 1c00a6cd0ff8c864f9e62f63d72289faedd9ece0
extra : source : 4dfb9fc82ca08cc41afc6b8e54026b9bb43658dc