Bug: incorrect computation of instrument limits

[mducle]

The function sw_instrument has code which computes the kinematic limits of the detectors. Unfortunately it assumes that the detectors span the range of scatterings [thetaMin, 180-thetaMin] and allows only a single argument for this computation thetaMin.

In fact, it is not physically feasible for most spectrometers to cover up to 180 degrees scattering angle. Thus a thetaMax parameter should be allowed and the expressions in the code modified. In practice most instruments cover around 120-130 degrees in scattering angle, and some cover an even more restricted range of around 60 degrees.