Include new classes to represent statistical data

[pantaray]

To adequately represent the result of averaging or pairwise connectivity analyses, a new class structure is needed. Similar to the already existing (abstract and concrete) classes, the following inheritance tree needs to be implemented:

graph TD
BaseData-->AnalogData;
BaseData-->SpectralData;
BaseData-->StatisticalData;
ABC-->StatisticalData;
AnalogData-->TimelockData;
StatisticalData-->|+avg +var +cov +dof|TimelockData
SpectralData-->FreqDescriptivesData;
StatisticalData-->|+avg +var +cov +dof| FreqDescriptivesData;
StatisticalData-->|+chan1 +chan2| ConnectivityData;
ConnectivityData-->|+freq| SpectralConnectivityData;

where StatisticalData is an (at the moment) empty abstract class. All other classes are instantiable and provide (at least) the following attributes (inheritance as indicated by above diagram):

TimelockData.avg
TimelockData.var
TimelockData.cov
TimelockData.dat

ConnectivityData.chan1
ConnectivityData.chan2

SpectralConnectivityData.freq

[joschaschmiedt]

The basic concept was implemented in 1a33fbd wihoutt any tests. The usable classes will follow in separate classes (#33 for TimelockData).