Explicitly floor start row/col and ceil end row/col

[sc1f]

This PR prevents to_format from being called with float start/end row and column indices, and unifies behavior across Javascript and Python bindings. This fixes an issue with perspective-datagrid where float start/end row indices were being sent to a perspective-python Table, and rejected by a C++ function whose parameters were integers.

For start_row and start_col, floor is called - so requesting a start row of 0.3 would start at 0, and not at 1. For end_row and end_col, ceil is called so that an end row of 1.2 would return 2, and not 1. This means that the data window should never be smaller than requested by the plugin.

[sc1f]

Fixes #1115

[texodus]

Looks good!

[texodus]

Thanks for the PR!