Fixes to `gridvolume` plugin

[diiigle]

gridvolume

• configure was not called from the default constructor
• the sine/cosine lookup tables were wrongly filled on the last element. (used in UInt8 data representation)
• the quantizedDirections data format requires only 2 channels

mtsutil

• add a utility class, that can generate constant-valued gridvolume files
• Fix a typo in an error message of test cases