Add support for the SUBTOTAL function: https://support.office.com/en-us/article/SUBTOTAL-function-7b027003-f060-4ade-9040-e478765b9939

It computes the subtotal for some ranges, given the code for the subtotal function:

SUBTOTAL(function_num,ref1,[ref2],...)

where function_num is code referring to some of the functions: AVERAGE, COUNT, COUNTA, MAX, MIN, PRODUCT, STDEV, STDEVP, SUM, VAR, VARP.

Note that we should implement the support for all of the above functions. As of now, COUNTA, VAR, VARP are not supported.

Workaround: This could be implemented as a custom function: http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/custom-functions.html , but multiple functions (4 as of now) has to be additionally implemented.