Fix set_parameter_path in Helmholtz Property Package

[bertkdowns]

Fixes

Fix the set_parameter_path() function in helmholtz.
update helmholtz_functions.py and helmholtz_state.py to use the latest cfg variable rather than storing a copy

Summary/Motivation:

the set_parameter_path function does not properly update the helmholtz parameter file path, because it sets idaes.properties.helmholtz.parameter_file_path rather than idaes.cfg.properties.parameter_file_path (note .cfg).

Also, helmholtz_functions.py makes a copy of the cfg variable into _data_dir, which also is not updated with set_parameter_path.

This involves needing to do some hacky workarounds to specify a different directory to load helmholtz data from (e.g see my register_compounds function.

Changes proposed in this PR:

• The _data_dir variable is replaced with a function that gets the config variable
• The set_parameter_path function is fixed to update the config variable correctly
• the compounds are auto-registered, as specified in the set_parameter_path documentation

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[dallan-keylogic]

@bertkdowns Could you please format your PR with Black? I'd like to see if it breaks the CI.

[bertkdowns]

@bertkdowns Could you please format your PR with Black? I'd like to see if it breaks the CI.

done, and the pytest tests passed on my machine, except for the ones that required the petsc solver as I didn't have it installed

[dallan-keylogic]

Looks like tests are passing, random Keras/multithreading errors nonwithstanding. @eslickj , if you get a chance, could you take a look to see if this breaks anything? At any rate, we should probably wait until after the 2.7 release to merge this.