similar to SamplePrimvar()

computed primvars.

Also added a VtValue specialization of HdResampleNeighbors(), for
resampling an HdTimeSampleArray of VtValue's.

If the value type can't be interpolated, just hold the previous sample's
value rather than producing a runtime warning.

This uses a simple computation to test the handling of time samples and
interpolating input samples.

…ghbors().

- Remove the USD dependency and just directly specify the types we
  expect to interpolate.
- Replace the BOOST_PP_SEQ_FOR_EACH usage with variadic templates.
- Fix some inconsistent code formatting.

This is similar to related methods such as SamplePrimvar().

This removes the hardcoded static capacity of 4, and templates
SampledValueStore to use the static capacity specified by the client
code.

Additionally, the internal _ExecuteSampledComputations() method is now
templated so that the static capacity of its value store arrays are also
controlled by the client.

Factored out some parts of the _ExecuteSampledComputations()
implementation that do not rely on the template parameter. This limits
the amount of code moved into the header (in particular, this also hides
the usage of internal classes such as Hd_ExtComputationContextInternal).