Implement formula for colored noise

[mgrub]

In uncertainty.propagate_filter.FIRuncFilter add possibility to change "color" of noise/uncertainty.

Remarks:

• This functionality seems not to be covered by the underlying paper: http://dx.doi.org/10.1088/0026-1394/45/4/013 .

Issue generated from in-code-TODO, see #41

[eichstaedtPTB]

In the paper cited here the equation (15) denotes the auto-covariance of the inputs x; equation (32) denotes the calculation of the uncertainties for x_low using the autocovariances (15). In the current code, though, only white noise with sigma_noise being of type float is considered.
Furthermore, for the output uncertainty U_y only the case of equation (33) is considered, but not (34).

[mgrub]

Draft Pull request: #55