Added tag v18.6 for changeset 6a929c3ad782
Created release v18.6
Added nearest_center()
updated docs
Added an overload of pick_initial_centers() that uses the linear kernel when no
other kernel is specified by the user.
Minor code cleanup
Added a value_type typedef to the array object.
Added another constructor to object_detector that makes it easy to combine
multiple detectors together.
Refactored the load_image_dataset() routines so they are easier to use and more
flexible.  This introduces a slight backwards incompatibility in that the
version that loads full_object_detection objects now returns an ignore
rectangle set instead of a parts name list.  But other than that the changes
are backwards compatible with previous version of dlib.
Added add_image_rotations() and also made add_image_left_right_flips() work
with full_object_detection objects in addition to rectangles.
updated docs
Clarified spec
Added another overload of extract_fhog_features().  This one returns the
features as a column vector but by reference instead of by value.
Made the asserts that checks if the inputs to the optimization functions are
column vectors always enabled since violating this is a common user error.
merged
The stopping condition for the svr_linear_trainer was too tight when
there were a lot of samples, causing it to take an excessive amount
of time to converge in some cases.  This fixes it.
Fixed a compile time bug that happened with gcc when DEBUG was #defined
but not ENABLE_ASSERTS.
Fixed typo in assert that prevented it from compiling
Gave pinv() an optional tolerance option.
merged