Address multiple neuron references pointing to same recording

[anilbey]

self.soma(0.5)._ref_v': Vector[2551] is same as neuron.h.cADpyr_L2TPC_bluecellulab_0x7f77e58f8a10[0].soma[0](0.5)._ref_v

(Pdb) self.cell.recordings
{'self.soma(0.5)._ref_v': Vector[2551], 'neuron.h._ref_t': Vector[2552], 'neuron.h.cADpyr_L2TPC_bluecellulab_0x7f77e58f8a10[0].soma[0](0.5)._ref_v': Vector[2553]}

Components to be affected: core.py test_core.py

Restrict the use of add_recordings instead use add_voltage_recording that takes the NEURON object as argument instead of recording str.