SPS OTFB and FF model

[htimko]

Updated SPS OTFB model with new features

• beam-induced voltage: replace semi-analytic model by signal processing model for consistency
• fine and coarse grid implementations of induced voltages
• in matrix convolution, use mode='full'
• turn-by-turn updating of parameters in update_variables()
• impulse response: rotation of arrays when off frequency
• reviewed sampling
• reviewed phase shift accumulation for beam-feedback compatibility (moved from beam_feedback to tracker)
• change order of delay and modulation
• class clean-up
• new unittests and updated benchmarks

New: optional embedded feed-forward

• FIR filter design routin
• filter design for SPS 3-, 4-, and 5- section cavities (fixed coefficients)
• optional enabling of FF in SPS OTFB on beam-induced voltage branch
• benchmarks and unittests

[coveralls]

Coverage increased (+0.6%) to 37.689% when pulling 04ceefceb54b8c8b3ec727eb77f9e6ddfec6a58e on htimko:spsff into 6780fcc15139a24dab8c81515b6f9931eff30963 on blond-admin:master.