fix bug in using multiGridParameterManager.
暂时使用Guo的外力处理方式以实现重力效果，但模拟过程中的尖角将导致计算过程发散。
new example collidingBubbles2d.
use negNiuDnvK instead of K as the parameter to avoid |K|=0.
Matrix2D rawTensor[2][2]-->>Matrix2D rawTensor.
more codes for 2d freesurface support
remove freeSurfaceTemplates2D.h.
rename freeSurfaceTemplates3D.h -->>freeSurfaceTemplates.h.
new files for bubbles opt.
more codes for multiFreeSurfaceModel2D
Merge branch 'develop'