Add query key population_type

[joni-herttuainen]

Added a query key population_type to query edge / node ids based on population type.

Examples:

# Nodes
circuit.nodes.ids({"population_type": "biophysical"})
circuit.nodes.get({"population_type": ["astrocyte", "biophysical"]}, properties=["x", "y", "z"])

# Edges
circuit.nodes.ids({"population_type": "electrical"})
# NOTE: circuit.edges.get can't be queried the same way as circuit.nodes.get

Closes #228

[joni-herttuainen]

# NOTE: circuit.edges.get can't be queried the same way as circuit.nodes.get

Actually, not quite true but we never claim that we'd support it:

circuit.edges.get({"population_type": "chemical"}) 
# returns: {"population_type": "chemical"} (dict)

circuit.edges.get({"population_type": "chemical", properties=["afferent_section_id"]})
# returns: pd.DataFrame with what you'd expect

Same happens with other queries used with Edges.get. Do we want to do something about this or leave it as is?

[codecov-commenter]

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@@            Coverage Diff            @@
##            master      #229   +/-   ##
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  Files           33        32    -1     
  Lines         2743      2712   -31     
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[mgeplf]

I think this is good to go, yeah? Seems reasonable to me, I think.