moved some often used methods in basic img classes to headers to allow inlining

modules/img/base/src/extent.cc

modules/img/base/src/extent.hh

modules/img/base/src/image_state/image_state_impl.cc

modules/img/base/src/image_state/image_state_impl.hh

...es/img/base/src/image_state/image_state_spatial_domain.hh

modules/img/base/src/image_state/index.cc

modules/img/base/src/image_state/index.hh

modules/img/base/src/image_state/value_holder.cc

modules/img/base/src/image_state/value_holder.hh