Disable first/last keywords

[corakingdon]

Currently on master you can't have components of different time lengths. The use of first and last keywords in add_comp! was disabled in order to get composite components working. I think before v1.0.0 we should decide if we are going to try to get this working, or if not, then I think we should remove the keywords from the add_comp! function all together.

[corakingdon]

If we won't allow this, then I think there are also some getindex functions we could remove, that were implemented for the case where the TIMES of the timestep don't match the TIMES of the timestep array, which would no longer be needed

[davidanthoff]

For now disable last and first keyword for 1.0.0.

[corakingdon]

delete keywords from add_comp function, and remove from documentation