[REQ] Maintain depth fix

[eon850aye]

@ andycaccia,

so things are looking up for the Type VIIb with regards to this rail look and keeping depth at slow speed.
It's not perfect, nothing ever is, but its getting there.
The boat has a nice pitch and roll on the surface now, and will maintain depth down to 200m. Unfortunately it is alittle shallow down to approx 110-112m, but only between 0.5-1m.

Last test results; speed at 2 knots
Depth Ordered.....Depth Achieved.....Depth Maintained
Periscope(12m)...........12m....................ye s
.....50m...................49.5m.................. .yes
.....88m....................87m................... .yes
....100m....................99m................... yes
....111m...................110m................... yes
....115m...................115m................... yes
....125m...................125m................... yes
....137m...................137m................... yes
....163m...................163m................... yes
....182m...................182m................... yes
....200m..................200.2m.................. yes

At 200m, when motors were switched off, you got that slow sinking as you mentioned before, but power on again and it stops and levels out.

So far I'm pleased with the results.

It still needs alittle fine tuning, and Im waiting to hear back from TheDarkWraith about something, but I will let you know as soon as I have something more solid for you to look at