propcov-cpp PointGroup: requires longitudes to be between -pi to +pi strictly

[vinay0000]

In the PointGroup class, need to modify the strict requirement on the range of longitude values. Currently the range is -pi to +pi.
While reading the grid file data, the PointGroup class filters out points not satisfying the above constraint.

See 35f5cf2

As long as this issue is there, the range of longitude values must fall within -pi to pi, else there shall be erroneous behavior, results.

Modify this behavior to also accept longitude values in the range 0 to 2pi.