Improved model_spike

[capelastegui]

Implement the following improvements for model_spike:

• cache variables, see #197
• new model: model_ramped_spike

model_spike

• review code - can make more similar to spike_date

model_spike_date

• review code - may be room for optimisation
• mask_spike - if is_mult, y=(A*mask_spike)+1.

model_ramped_spike:

• single param A
• Additional params, only used fixed
• date_start_ramp, date_start, date_end_ramp, date_end
• otherwise same as model_spike_date

mask_spike =

• ramp from 0 to 1 between date_start_ramp, date_start
• 1 if date between (date_start, date_end)
• ramp from 1 to 0 between date_end, date_end_ramp