initial sketch of how tests for matrix 2d might work.
Fixed old alpha-renaming in Cursor.gather (bufferOffset, not bufoffset).

Added directive to callsite-clone Cursor.gather.

Self-review uncovered oversight: Cursor does not carry valtype, so the
MatrixPFill check that the expected valtype matches the returned
valtype was going to fail on Cursor.gather.  "Fixed" by allowing one
to skip the expected valtype check; but that is not really good.
initial draft of 2d matrix test leveraging out ptr to work around nested cxn issue.
pre-alpha draft of matrix variant of nbody.