Strutt|Transmission Loss|Composite TL calculates the composite transmission loss of a partition, given the areas and transmission losses of the wall components, and inserts the results into the active row of the worksheet.
Strutt checks to see if the TLs are all positive and all negative (and returns an error if they are mixed!) and prompts the user to enter the area of each element in Column D of its row.
For positive TLs, the formula is implemented as:
`TL_(ave(+)) = 10log_10((sum S_i)/(sum S_i 10^(-TL_i//10)))`
For negative TLS (Strutt's sign convention), the formula is implemented as:
`TL_(ave(-)) = -10log_10((sum S_i)/(sum S_i 10^(TL_i//10)))`