cell2underlying for tall arrays

Simplifies use of tall arrays with fileDatastore

When you make a tall array from a fileDatastore, the resulting tall array is a tall cell array. CELL2UNDERLYING converts that tall cell array to a tall array of the underlying data. For example, if your fileDatastore returns a table for each file that it reads, then a tall array based on that fileDatastore would be a tall cell array where each cell contained a table. Using CELL2UNDERLYING would convert that to a tall table.