FT_MATH performs mathematical operations on FieldTrip data structures,
 such as addition, subtraction, division, etc.

 Use as
   data = ft_math(cfg, data1, data2, ...)
 with one or multiple FieldTrip data structures as the input and the configuration
 structure cfg in which you specify the mathematical operation that is to be
 executed on the desired parameter from the data
   cfg.parameter = string, field from the input data on which the operation is
                   performed, e.g. 'pow' or 'avg'
   cfg.operation = string, for example '(x1-x2)/(x1+x2)' or 'x1/6'

 In the specification of the mathematical operation, x1 is the parameter obtained
 from the first input data structure, x2 from the second, etc.

 Rather than specifying the operation as a string that is evaluated, you can also
 specify it as a single operation. The advantage is that it is computed faster.
    cfg.operation = string, can be 'add', 'subtract', 'divide', 'multiply', 'log10', 'abs'
 If you specify only a single input data structure and the operation is 'add',
 'subtract', 'divide' or 'multiply', the configuration should also contain:
   cfg.scalar    = scalar value to be used in the operation

 The operation 'add' is implemented as follows
   y = x1 + x2 + ....
 if you specify multiple input arguments, or as
   y = x1 + s
 if you specify one input argument and a scalar value.

 The operation 'subtract' is implemented as follows
   y = x1 - x2 - ....
 if you specify multiple input arguments, or as
   y = x1 - s
 if you specify one input argument and a scalar value.

 The operation 'divide' is implemented as follows
   y = x1 ./ x2
 if you specify two input arguments, or as
   y = x1 / s
 if you specify one input argument and a scalar value.

 The operation 'multiply' is implemented as follows
   y = x1 .* x2
 if you specify two input arguments, or as
   y = x1 * s
 if you specify one input argument and a scalar value.

 To facilitate data-handling and distributed computing you can use
   cfg.inputfile   =  ...
   cfg.outputfile  =  ...
 If you specify one of these (or both) the input data will be read from a *.mat
 file on disk and/or the output data will be written to a *.mat file. These mat
 files should contain only a single variable, corresponding with the
 input/output structure.

 See also FT_DATATYPE