Added a placeholder for Double extension methods

To store helper stuff and also isolate Math.NET Numerics.
Implemented DoubleExtensions methods
Added XML comments to the DoubleExtensions. Method renaming
Fixed a typo
Replaced Math.NET Numerics double extensions with own ones

Again, this is to isolate Math.NET Numerics - in case I decide to take
another library or create my own implementations of necessary methods.
Updated ISparkGenerator interface. Implemented missing logic in ExplosionSparkGenerator
Added SparkGenerator class that is to be used as base for spark generators (at least, for some of them)
Inherited ExplosionSparkGenerator from SparkGenerator
Added IContinuousDistribution interface
Added NormalDistribution

For now, it is just a wrapper around Math.NET Numerics Normal class.
Exception arg typo fix
Moved Gaussian spark generation logic to GaussianSparkGenerator class
XML comment fix
Cosmetics