Define serialization transition period length and prepare removing old routines

[lumip]

We must define a transition period during which the old routines are deprecated but still available and, preferably, already prepare a branch where they are removed which is to be merged after the transition period expires.

[terrorfisch]

We should be able to load all old pulses anytime.

We should however remove the old writing routines. Maybe by the end of the year?

[terrorfisch]

The old writing routines will be dropped on the 1.0 release. They will be deprecated in the next release.

[terrorfisch]

They are already deprecated.