Allow number of untrapped pitch angles ng2 to be set directly in the input file.

If ng2 not set then it defaults to 2*ngauss in order to maintain backwards compatibility.

The primary motivation for this is to improve the availability of helpful sweet spots. As the number of trapped pitch angles is odd, the total number of pitch angles will be odd with the old behaviour (even ng2). By allowing odd ng2 users can tweak their pitch angle resolution to open up more useful even sweet spots.

Addresses issue #136