Gilbert Elliot Channel Model

[kunterbunt]

Besides some bug fixes and additional statistic collection, this branch implements a new channel model: the abstract two-state Gilbert-Elliot Hidden Markov Model.

Configuration and usage is simple using

**.lteNic.LteChannelModelType = "LteGilbertElliotChannelModel"
**.lteNic.channelModel.trans_prob_bad_state = 0.5 # p21
**.lteNic.channelModel.trans_prob_good_state = 0.25 # p12
**.lteNic.channelModel.error_prob_bad_state = 1.0
**.lteNic.channelModel.error_prob_good_state = 0.1

where transition probabilities from a good to a bad state can be specified and vice-versa, as well as their error probabilities. The discrete-time model is evaluated everytime the error function is used: a random number determines if the channel state changes, and the current state's error probability determines whether the current packet is lost or not.