* ses.el: Use hash map for getting named cells coordinates.
symbol to coordinate mapping is made by symbol property
`ses-cell'. This means that the same mapping is done for all SES
sheets. That is good enough for cells with standard A1 names, but
not for named cell. So a hash map is added for those
latter.
(defconst ses-localvars): added local variable ses--named-cell-hashmap
(ses-sym-rowcol): Use hashmap for named cell.
(ses-is-cell-sym-p): New defun.
(ses-decode-cell-symbol): New defun.
(ses-create-cell-variable): Add cell to hashmap when name is not A1-like.
(ses-rename-cell): Check that cell new name is not already in
spreadsheet with the use of ses-is-cell-sym-p
(ses-rename-cell): Use hash map for named cells, but accept also
renaming back to A1-like.