Add cpubasedvolume to GR module

[kojix2]

　This doesn't sound too difficult, but there is one problem. That is, the GR module depends on Numo::NArray. Until now, GR has been written to be independent of Numo::NArray. (The GR3 module is different. It relies entirely on NArray from the start). This is because I want to be able to use the GR modules even in environments where we cannot install the development tools. Methods such as cpubasedvolume should only take narray as an argument. There is a difficult problem to solve, #27.
　I love narrays, so it is possible to make all GR.rb depend on NArray. This might make GR cleaner. I need more time and energy to make this decision.