Make Parameter a Template

Make parameter a template, such that it can support other types than double as well; see TODO in its doxygen comment:

/**
 * @brief Class for Filter-parameters
 *
 * This class is made to hold parameters for filters.
 * It has a minimum and maximum value and notifies the
 * associated filter of changes.
 *
 * @todo make a template out of it, so different types are supported
 */
class Parameter
{
private:
    double  mValue;
    double  mMinimum;
    double  mMaximum;
    bool    mChg;
    Filter *mFilter; // Filter where parameter is for