Use default values for properties of DesignSource and DesignElement

[ichinii]

for most properties, there is a expected default behavior, which often is the desired behavior. this makes specifying properties optional, when creating objects algorithmically

for example

-DesignSource::horizontalDivergence describes random distribution of x component of ray direction
an expected default value for the angle would be 0 resulting in: no random distribution at all