Full unit safety

[ikicic]

At the moment, support for pint and unit-qualified values is rudimentary. Mirheo knows how to transform Quantity objects to Mirheo's units, but it does not know if the dimensionality is correct. Concretely, when Mirheo receives a Quantity as an argument, it transforms it to the (mirL, mirT, mirM) unit system, takes the magnitude and discards the dimension.
This leads to hard to find bugs, e.g.: mole is not converted to N_avogadro, the fact that mesh scale depends on mirL may be forgotten etc.

We should find a way to add strict unit checking. To make that possible, either Python or the pybind C++ code somehow has to know the dimensionalities of all parameters of all APIs. The most robust solution I can think of is to replace real parameters (in bindings, not in the core) with Length, Duration, Energy and other similar types, each representing one dimensionality. Then we somehow define implicit conversions from pint Quantity to these types.