[r] Coords handling for writing dense subarrays

[johnkerl]

As discovered on #2407: we found that R SOMADenseArray's write ignores the coords passed to it.

This means a user cannot have, say, a 1000x20000 dense array and write portions of it. This is important for Seurat work being done by @mojaveazure .

To be also done on this issue:

• Unit-test 0-up and 1-up coordinate handling
• Given all the new unit tests on the Python side for [python] Dense subarray-write full corner handling #3271, implement them all in R as well
• Add cross-language tests in apis/system/tests to test R write and Python read, and vice versa