Observability: internal voltage count check

[nitbharambe]

Relates to #508

There was a bug in voltage measurement observability check. By the virture of conditions, the situation never results in a incorrect behavior and the tests cover these cases.

Earlier (naming convention used as per issue here):

if (n_voltage_sensor + n_voltage_phasor_sensor < 1):
	throw NotObservableError{}

Corrected:

if (n_voltage_sensor < 1):
	throw NotObservableError{}

n_voltage_sensor	n_voltage_magnitude_sensor	n_voltage_phasor_sensor	result
2	1	1	pass both before/after
1	0	1	(never possible)
1	1	0	throws both before/after
0	0	0	throws both before/after

Added a fix, renamed to how it is in issue to avoid ambiguity.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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