Introduce namespace `adaflo`

[mschreter]

In a student project, we use adaflo and the deal.II TimeStepping class (https://www.dealii.org/current/doxygen/deal.II/classTimeStepping_1_1TimeStepping.html). Since adaflo also has a TimeStepping class (see https://github.com/kronbichler/adaflo/blob/master/include/adaflo/time_stepping.h), we get a naming collision. @kronbichler would you agree, if we introduced an adaflo namespace around the whole code? It's a big incompatibility change, but it would help avoid naming collisions in the future.

@j-brotz FYI