Incomplete fix of editor mass discrepancies

The editor shows incorrect mass information for tanks in the new "i" toolbar button, like what is shown here: https://imgur.com/ahra20t.  This tries to fix the masses being out of whack, but suffers from an update delay problem where the toolbar shows the last-edited state of the tank (maybe some kind of race condition on the editor update call?)  Hopefully will be of some use.

Note the RealFuels section is totally untested.
TankContentSwitcher now calls onEditorShipModified event after receiving a VolumeChange event (fixes problem in PR #87 )
removed deprecated class TankContentSwitcherRealFuels
IPartMassModifier interface added to DecouplerTweaker
deleted folder PartsDeprecated since it is no longer needed
set the mass of every part (except Real Fuel parts) to 0.
merge pull request #87 into master