DataTable is backed by SimTK::Matrix which is intended for only composite numerical types

[aymanhab]

While users can make DataTable(s) of all kinds of objects (Quaternions, UnitVecs, Rotations), the data-structure that backs the DataTable is a SimTK::Matrix which only supports Composite Numerical Types, per discussion in PR #2467

[aymanhab]

Composite Numerical Types are supported, so we can use DataTable and DataTable underneath, and then make a subclass that provide convenience/type-checking methods on top of that so we're not violating the assumptions built into SimTK::Matrix. Alternatively we can back the DataTable by a SimTK::Array but that seems more drastic change.

[aseth1]

@sherm1 and @aymanhab. There still isn't any intention, desire or need to use Matrix_<Rotation> or Matrix_<Quaternion> for composite math. Matrix_ is handy for backing Tables because of the ease of access by row, column, element and submatrix. A similar discussion came up several years ago with Shrinidihi when we chose Matrix_ to back DataTable_. The consensus then was that we'd cross that bridge if/when we got there with non composite types. If we can get away with just using it as container and enabling element access via scripting, then I agree with @aymanhab that rebacking with SimTK::Array is not really nececessary.