Squashed commit of the following:

commit 510b9353f8cb932a87f1588b17a4dea230c29d3c
Author: mattijs <mattijs>
Date:   Wed Jun 14 16:36:35 2017 +0100

    ENH: timeControl: propagate time-change logic from Time.C

commit 6dc57a8f1e0e7605ea819deb6f11dd4b7874ff30
Author: mattijs <mattijs>
Date:   Thu Jun 1 11:28:56 2017 +0100

    ENH: timeControl: cleanup; avoid division by zero

commit 5ac4bc2dd8c1f4676eef9d7a03215caba23a2e19
Author: mattijs <mattijs>
Date:   Wed May 31 12:04:52 2017 +0100

    ENH: timeControl: allow ramping down as well as up.

commit a6b2db9e791f29258f04f3a9cbd6354aa468977d
Author: mattijs <mattijs>
Date:   Wed May 17 15:29:22 2017 +0100

    ENH: timeControl: limit any timestep change if deltaTCoeff enabled.

commit 004115ee03a6637ae0d23cce303a30d1b3af046f
Author: mattijs <mattijs>
Date:   Wed May 17 11:40:26 2017 +0100

    ENH: setTimeStep: have timeStart, timeEnd controls on time step adjustment.

    Also added smoothly varying time step change (through optional deltaTCoeff)