Marked the PTC_STATE_SIMULATION_VALID flag as deprecated. This is only
used by the bake operator, but keeping track of the sync state of cache
data vs. simulation settings needs to be radically simplified.

The API should provide a way of detecting whether a valid simulation
state can be obtained for a certain frame.

  /* check if a frame can be read from the cache (no data is modified) */
  PTCReadSampleResult PTC_test_sample(struct PTCReader *reader, float frame);
  /* try to obtain a cache sample and write to DNA data */
  PTCReadSampleResult PTC_read_sample(struct PTCReader *reader, float frame);

Beyond that the cache should be oblivious of the simulation state. When
settings or dependencies are changed which invalidate the simulation
cache state this should invalidate all cache samples (or possible a
certain frame range), but the cache itself should not need to keep track
of the simulation as a whole.
Completely removed the "bake" operators for point cache. This feature
will get a complete replacement, removing it altogether makes this much
easier.
Removed the SIMULATION_VALID flag entirely. This was only used by the
bake operators.
Removed the BAKED flag from point cache state. The cached now has a new
flag LOCK_SETTINGS instead, which should indicate more clearly that it
is about preventing cache modification by disabling user settings
changes.

The cache should not have to care about this setting in the first place.
It becomes increasingly difficult to control changed simulation settings
anyway (scripting!), and the cache should not even try to exert control
over sims this way. Eventually this flag should be a feature of
simulations and modifiers, using it as a 3rd option beside
enabled/disabled and using only existing cached data.
Added XXX comment about the BAKING flag, left it in for now to not lose
track of the purpose.
Moved object/modifier/sim resets out of the BKE_ptcache_id_reset
function. This is a design cleanup, the point cache methods should only
write to the DNA when retrieving state data. There were already a number
of cases where such resets occured outside of this function (such as
with rigid body worlds or smoke sims).

The BKE_ptcache_object_reset function still exists and includes these
reset calls, but there the cache user data types are accessed directly.