method to estimate base request size #263
expose data types on dimension objects #265

Added dimension sizes to the dictionary return. The mapping has
changed. Also need to add a dtype property to the VectorDimensions.
This required some modifications to the inheritance structure.

Closes #263.
Closes #265.
Merge branch 'master' into i243-large-array
mark - before changing SpatialCollection inheritance
mark - all tests working
mark - minor