Clean CP curve

.. when in range mode (i.e. plotting a CP curve for a date
   range or season rather than for a ride).
Reinstate 3 point CP model

DJ Connel implemented a 3 point model, after Morton,
which was erroneously removed in a prior commit.

Our thanks to Andy Coggan for pointing this out in a
recent webinar. Nice to know he is following us closely.
User Definable CP Model

Let the user define whether to use a 2 point or
3 point model (Morton et al).

In addition, let the user define the interval durations
to use; by default we set to 3 and 30 mins.

For estimating CP against good, recent data both of these
models are more than adequate for the purpose.

We'll just keep kicking the can down the road whilst we
can rely on published, verified science.
The CP Model has "parameters" not "points"

Might as well use the right terminology if your gonna
let the user choose and adjust the model!
Estimate CP and W' in CP Estimator

Since we want to track W' and CP now it makes sense
to add W' estimation to the tool.
CP Curve tmin

.. accidentally left at 0s now corrected back
   to truncate at tau for the 2 parameter model
   and set to 25s for the 3 parameter mode where
   it seems to breakdown.
Don't truncate 3 parameter CP curve