Fix a bug in IceModelVec3Custom. Port to 0.6.
IceModelVec2T does not re-read periodic data. Port to 0.6,

Periodic data is read in once at the beginning of the run (in
IceModelVec2T::init()), so we don't need to check if everything we need
is held in memory (we know that it is).
Replace 1 with 1.0 so that types match.
Fix -ocean ...,cache. Port to 0.6.
Fix -surface ...,cache. See #254. Port to 0.6.