AP_Math: add longitude_scale comment & rename variable
AP_InertialNav: add comments, rename incorrectly named member,
initialize member, remove redundant assignment
AP_InertialNav: remove unused AP_InertialSensor pointer
Copter: add comment (unit of G_Dt)
AP_Baro: add time-unit comment
Filter: add consts
Filter: add virtual specifier in subclass to make code clearer and
easier to follow by other developers
Filter: remove useless virtual specifiers, add const

Almost all methods were specified as virtual, although they didn't
override anything or weren't overridden.

The only methods that are actually overridden are the ones from the base
class Filter:
    virtual T apply(T sample) = 0;
    virtual void reset()  = 0;
    
This saves 982 bytes for the Copter code (apm2-quad)
AP_AHRS: initialize member
AP_Baro: add consts (mutable, virtual)

* add virtual specifiers to overriding methods in subclasses to make it
clearer that they are virtual too.

* the mutable specifier was added to some private members that are only
used for caching results. this was done to allow some getters to be
const.
a getter usually simply (calculates and) returns a value and doesn't
change the object's (visible) state. this expected behaviour should be
reflected in the getter's signature.
AC_Sprayer: add consts, update comments

* removed some comments that were left over from the classes this class
was copypasta'ed from
Copter: Log.pde: replace unused copy with ref (saves again some bytes)
AC_Sprayer: replace unnecessary object with const ref
AC_WPNav: replace unnecessary objects with const refs
AP_InertialNav: add comments & function; restructure

* added documentation to all methods (We tried to understand the
internals of this class from the prior given comments and document it as
far as possible)
* "set_current_position" was renamed to "set_home_location" to reflect
it's intended behaviour
* a new member (_position) was introduced that simply caches a sum
frequently used by getters (saves some space)
* some consts were added
* added method for changing the current position estimate (needed for
future mavlink state correction)
Copter: commands.pde: accommodate for changes in AP_InertialNav