Analyser: allow for strict or relaxed units checking

[agarny]

We should support two levels of units checking:

• strict: issues found with units are considered as errors; and
• relaxed: issues found with units are considered as warnings.

By default, strict units checking would be used, this to encourage people to create "good" models.

[agarny]

This model has various issues with units. Yet, libCellML is fine with it. Some of the issues include:

• dV/dt = f_a - k_e * V with units of per_ms = per_s - dimensionless * dimensionless; and
• sin(10 * t) with units of sin(dimensionless * second).

This model could be used to test things for this issue.