In particular, use this feature in many loops which feature a special treatment
for the first iteration, using a boolean variable (replacing iterator
manipulation). By the way, also use std::exchange() to simultaneously test the
value of this variable and update it.

Add tolx-option for steady

See merge request !60

And, symmetrically, when the “bytecode” option is requested by the user, always
create the .m static/dynamic files.

The “bytecode” option therefore no longer modifies the preprocessor output.

Closes: #97

This limits the risk of slicing.

The same is not possible for DataTree, since that base class is directly used
in some places.

The vectors were previously stored as strings internally, which would not
necessarily contain the commas which are required in JSON (but not in
MATLAB/Octave). The fix consists in internally storing those options as
std::vector.

Closes: #96

However, disable -Wunused-parameter which creates too many false positives in
polymorphic methods.

Also, only disable -Wdangling-else instead of the whole of -Wparentheses.

We follow the advice given by Josuttis in his book about move semantics.

Deleting those member fuctions can be a bug if we want to allow copy semantics,
because overload resolution will no longer fallback to the copy
constructor/assignment operator when given an rvalue.

In particular, this explain why it was not possible to delete the move
assignment operator of the StaticModel class.

This disables warnings when -Wimplicit-fallthrough is used (enabled by -Wextra).

By the way, remove the unused TrendComponentModelTable::getOrigDiffVar() method.

C++17 modernization: use std::optional for the storage of orig_symb_id and orig_lead_lag in SymbolTable

For the diff and unaryOp auxvar types, these value may be set or unset
depending on the complexity of the expression represented by the auxvar.

Incidentally, in the JSON output, no longer assign a value to the “periods”
field if not passed by the user (previously the value -1 was output in the JSON
file).