Ability to input junctions at `ProjectInfo` initialisation

[nikosavola]

Similarly to #103, I think junctions should be possible to give in ProjectInfo initialisation as arguments.
Doing

pinfo.junctions['jAlice'] = {'Lj_variable':'Lj_alice', 'rect':'rect_alice', 'line': 'line_alice', 'Cj_variable':'Cj_alice'}
pinfo.junctions['jBob']   = {'Lj_variable':'Lj_bob', 'rect':'rect_bob',  'line': 'line_bob', 'Cj_variable':'Cj_bob'}

feels similarly hacky to #102 (though maybe not as much).

A syntax with dictionary of dictionaries could be supported

junctions = {
    'jAlice': {'Lj_variable':'Lj_alice', 'rect':'rect_alice', 'line': 'line_alice', 'Cj_variable':'Cj_alice'},
    'jBob': {'Lj_variable':'Lj_bob', 'rect':'rect_bob',  'line': 'line_bob', 'Cj_variable':'Cj_bob'}
}
pinfo = epr.ProjectInfo(..., junctions=junctions)

In this case pinfo.validate_junction_info() could be called automatically and an Exception raised

[zlatko-minev]

This would be backwards compatible?