* [CORE] allow updating trees in an existing model
* [CORE] in refresh updater, allow keeping old leaf values and update stats only
* [R-package] xgb.train mod to allow updating trees in an existing model
* [R-package] added check for nrounds when is_update
* [CORE] merge parameter declaration changes; unify their code style
* [CORE] move the update-process trees initialization to Configure; rename default process_type to 'default'; fix the trees and trees_to_update sizes comparison check
* [R-package] unit tests for the update process type
* [DOC] documentation for process_type parameter; improved docs for updater, Gamma and Tweedie; added some parameter aliases; metrics indentation and some were non-documented
* fix my sloppy merge conflict resolutions
* [CORE] add a TreeProcessType enum
* whitespace fix
* Fix various typos
* Add override to functions that are overridden
gcc gives warnings about functions that are being overridden by not
being marked as oveirridden. This fixes it.
* Use bst_float consistently
Use bst_float for all the variables that involve weight,
leaf value, gradient, hessian, gain, loss_chg, predictions,
base_margin, feature values.
In some cases, when due to additions and so on the value can
take a larger value, double is used.
This ensures that type conversions are minimal and reduces loss of
precision.
* [TREE] Experimental version of monotone constraint
* Allow default detection of montone option
* loose the condition of strict check
* Update gbtree.cc
