Add reference to constructing class in ProcessBlock

[bknueven]

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Fixes N/A

Summary/Motivation:

As best as I can tell, I cannot easily get the constructing class back from an instance of a process block. E.g.

from pyomo.environ import ConcreteModel, Var, value
from pyomo.common.config import ConfigValue
from idaes.core import ProcessBlockData, declare_process_block_class


@declare_process_block_class("MyBlock")
class MyBlockData(ProcessBlockData):
    CONFIG = ProcessBlockData.CONFIG()
    CONFIG.declare("xinit", ConfigValue(default=1001, domain=float))
    CONFIG.declare("yinit", ConfigValue(default=1002, domain=float))

    def build(self):
        super(MyBlockData, self).build()
        self.x = Var(initialize=self.config.xinit)
        self.y = Var(initialize=self.config.yinit)


m = ConcreteModel()
m.b = MyBlock()

# how to get a link back to `MyBlock` from `m.b`?
# m.b.__class__ is <class 'idaes.core.base.process_block._ScalarMyBlock'>

I have a use case in WaterTAP which would aggregate some results by unit model type. Ideally we would index by the creating class, e.g., MyBlock in this example, but it does not seem to be possible to get this class back from an instance of _ScalarMyBlock or MyBlockData.

Changes proposed in this PR:

• Add function process_block_class to the _ScalarProcessBlockMeta and _IndexedProcessBlockMeta metaclasses
• Add tests for the new metho

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