Ground work for supporting writable cells
Ground work for supporting writable cells

Added more explicit exceptions.
Cleaned up the way Columns are obtained by index on Rows.
Added method to fetch a Cell on a Row by Column ID.
Added stubbed out method to set a Cell's value.  There's a lot left to
do prior to making Cells writable, but it's a start.

smartsheet-platform/smartsheet-python-sdk,smartsheet-platform/smartsheet-python-sdk