Fix indigo pka model API. Remove static model

Remove static initialization in pka model (molecule_ionize.cpp)
- create molecule_pka_model.cpp and move MoleculePkaModel class
- change indigo methods signatures

// Method for building PKA standard model
CEXPORT int indigoBuildPkaModel (int max_level, float threshold);

// Method for building PKA custom model from user SD file
CEXPORT int indigoBuildCustomPkaModel (int max_level, float threshold, const char *filename);

//use standard model
CEXPORT int indigoIonize (int item, float pH, float pH_toll);
CEXPORT int indigoIonizeCustom (int model, int item, float pH, float pH_toll);

CEXPORT float \* indigoGetAcidPkaValue (int model, int item, int atom, int level, int min_level);
CEXPORT float \* indigoGetBasicPkaValue (int model, int item, int atom, int level, int min_level);