Parse Gaussian Mixture Model prior in lalapps_knope pipeline configuration file

The lalapps_pulsar_parameter_estimation_nested code can take priors that are Gaussian Mixture Models (GMM) (i.e., the sum of several Gaussians with different means, variances and relative weights). However, the lalapps_knope pipeline code was not able to parse the required parameters for a GMM prior from a configuration file. This patch now allows for GMM prior parameters to be parsed and correctly output into the prior file required by lalapps_pulsar_parameter_estimation_nested.

Additional minor changes:

• Some outdated documentation on the style of the GMM prior file input for lalapps_pulsar_parameter_estimation_nested has been corrected

cc @david-keitel @karl-wette