FieldConvert module for combining average fields

This MR introduces a new FieldConvert module that combines fields obtained through FilterAverageFields or FilterReynoldsStresses (the Reynolds stresses are added correctly, what is not possible using only addfld).

The number of samples in each field is automatically considered and the total number of samples is stored in the output file, allowing for multiple files from simulations with different numsteps to be combined easily.