Implement support for reading CST beam files with phi ranging from -180 to 180.

[mickeyhorn]

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In the default configuration for CST's exported beam files, the phi/azimuthal angle ranges from 0 to 360 degrees. There is an option to export it as -180 to 180 degrees, and ECHO would like to do so for our current analysis. Would this be possible to implement?

[bhazelton]

I actually think it might be supported already, from a quick read of the code. What isn't currently supported is changing from 180 to 180 to 0 to 360 -- the code just leaves it as is and the user would need to makes sure they asked for angles in the appropriate range for e.g. interpolation calls.