Implement "is_initialized" method in constituents object

[peverwhee]

Description

Implement a way to query the constituents object for whether or not a given constituent has been initialized (is not the default value).

Solution

Implement is_initialized procedure within ccpp_model_constituents_t that will return true if any element in vars_layer(:,:,const_idx) for a given constiuent index or standard name, is not equal to the default value.

Capgen version

This should be applied to both v0 and v1 if implemented before unification.