Naming scheme for dimensions in function headers

[hagenw]

As discussed in #141 we have some inconsistencies in the description of dimension length in the function header.
For example

%       x0      - position  and direction of secondary sources [N0x7] / m
%       d       - matrix of driving signals [NxN0]

in driving_function_imp_nfchoa() as opposed to

%       x0      - positions and directions of secondary sources / m [nx6]
%       d       - driving signals [mxn]

in driving_function_imp_wfs().

We should agree on a common naming scheme and create a pull request in order to fix this.