Loading *.gnumeric tables can be slow and use a lot of memory

[kasemir]

.. if those files claim to contain 65000 rows and many columns, even if there are actually only very few non-empty cells.

This can happen when for example formatting a column in Gnumeric, which results in all rows having some metadata.

Need to update python code to only fetch valid cells, then create table just for those cells, instead of starting with 65000 row table, then trying to cull empty rows and columns.

[kasemir]

Fixed by 5f67dfc2129d49df3b1897ede7695c759b34dd3c