1. Information about a unit's dimension now uses inheritance.
_m is an instance of DistanceAlias, which is derived from Alias.
A UnitNode now keeps a pointer to an Alias and one to a Prefix.
All aliases are still defined as constexpr.
This cleans up a lot of the code used namely for computing the
additional outputs in Calculation.
2. Instead of being defined with a string, each unit is described by its
ratio with the base SI unit (ex: _L is 0.001 instead of "0.001_m^3").
This greatly speeds up the calculations using units, as the algorithm
to find the best unit used to parse the definition.
Change-Id: I4d6ed6ad4cb967026a3f01a335aec270066e2b9f
If an expression hasUnits and is then reduced, it might not have units
anymore for instance if it was replaced with undefined).
Scenario: Enter "[5000000000000000]^20 _s" in the calculation app
computed when the calculation is added to the store and don't change afterwards.
Otherwise, if their heights change when scrolling (due to a modification of the
display output type - ExactOnly, ApproximateOnly...), it generates crashes.
calculation height memoization. They're computed from the layouts which
don't depend on the complex format (or any other settings parameters
which could have changed).
When each Calculation had its own text buffer, the Expression of an
overflowing output text was Undefined. Hence that Expression could have
been Undefined without the corresponding text being 'undef'. That situation
may not happen anymore since Calculation are stored in a shared buffer.
The most common case where only the approximate output is displayed
happens when the exact and the approximate outputs are equal, more
precisely when the exact output is equal to the approximate output with
the number of significant digits taken from user settings. That
clarification matters for the Poincare::Integers which have more digits than
the number of significant digits set by the user.
Besides, an Expression containing Units is simplified as
Multiplication(Float, Unit...) recorded identically into the exact and
the approximate outputs (except perhaps the number of significant
digits). Generally, the case where the number of digits in the output is
smaller or equal to the user setting would be matched by the condition,
stated above, that exact and approximate output are equal. But that
should also be the case whatever the number of significant digits. That
is why the presence of Units is matched specifically.
and 2 approximate outputs - one with the maximal number of significant
digits and one with the number of significant digits asked by the user.
This enables to find the approximate output without going through the
approximation routine again.
