Node size range value should be calculated from min/max value of the given parameter

[keiono]

Node size mapping is set as a static value in this version. Instead, set range based on the given statistics. For example, if degree is set for node size mapping, min and max of degree should be the range.

[ray0bump0]

I changed node size mapping to depend on the given statistics.
In addition, I also changed meaning of "maxnodesize" that was max range value to the value of max node size.