Enhancements for fields

[ahojukka5]

It would make sense to add some extra information for fields. I have some ideas:

• to show that should all components of a vector or tensor field be active, we need some flag (see issue Performance problem with interpolate #47)
• some fields may depend on time but might still not be possible to interpolate at least with linear interpolation, we should have some options to define how to interpolate in a time direction (nearest neighbor, linear, quadratic) and also how to extrapolate, when outside of time span
• possibility to store "dual" variables needed to perform AD
• possibility to store "increment" variables, i.e. if we have ´displacement`, but we actually solve increment \delta u, which is then added to displacement, u_t = u_{t-1} + \delta u, when converged