Iterator for a variable at all times in Collocation class

[drewm1980]

It would be nice if the collocation class had a built-in iterator that returns the variable over the whole range...

i.e. instead of something like:

obj = 0
for k in range(N):
for j in range(ocp.deg+1):
obj += ocp('abspos',timestep=k,degIdx=j)
obj += ocp('abspos',timestep=N,degIdx=0)

user does something like:

obj = 0
for abspos in ocp.timeIteratorForVariable('abspos'):
obj += abspos

or maybe ocp.iterValueForAllTimes('abspos')

perhaps the time should be returned too, i.e.:

obj = 0
for (abspos,t) in ocp.timeIteratorForVariable('abspos'):
obj += someTimeVaryingFunction(abspos,t)

[ghorn]

See also #6

One way would be for
for pos in ocp('pos')
to loop over timestep, degIdx, intermediateControlIdx
for pos in ocp('pos',timestep=5)
to loop over degIdx, intermediateControlIdx
etc

Maybe it would be easier to just have ocp('pos') to return an ndarray that the user could slice manually. But even then, some variables are undefined at some nodes, for example algebraic variables are undefined at degIdx=0, nothing is defined at timestep=N except differential states, nothing at all is defined for timestepN, degIdx>0 or intermediateControlIdx>0

This is not so simple to handle generally but also intuitively for the user. Maybe this functionality should be pushed into a higher level Ocp class.