unification of time string parsing

[Joern Ungermann]

Both the server and the client need to convert iso-formatted strings into datetime objects. This is done inconsistently and in the case of the client in a rather hacky way.

This should be unified using the isodate or dateutil python packages.

[Reimar Bauer]

We remove python-dateutil from our package list

[Reimar Bauer]

Consistent use of isodate.

Simplification of date parsing. Everything now uses isoformat and we use only a single method based on isodate for conversion from strings to datetime and back. pytest passes as well.

Fix issue #440

→ <<cset 8c2929f5d149>>

[Reimar Bauer]

Fix problem in isoformatted dates not being accepted.

See issue #440

→ <<cset 78274d7e544d>>