Add alternativeName() for symbols

[jlblancoc]

What do you think of this feature?

It allows writing code with symbol named according to their physical meaning. Check the example in the new unit test.

Of course, the default KeyFormatter would still return the underlying one-character symbol name, but if someone want to have custom names printed, a custom KeyFormatter would be required.

[dellaert]

That's cool. But it took a while before I figured out what was going on :-) I think a functor might be easier to grok than a function producing a function. E.g.

gtsam:SymbolGenerator Z('x'), DZ('v'), DDZ('a');

implemented like

class SymbolGenerator {
const char c_;
public:
SymbolGenerator(const char c) c_(c) {} 
Symbol operator()(const std::uint64_t j) const { return Symbol(c_, j); }
};

What do you think?

[jlblancoc]

Updated!
One remaining doubt: what would be better, gtsam::symbol_shorthand::SymbolGenerator or gtsam::SymbolGenerator ?

[jlblancoc]

PS: If it's the shorter gtsam:SymbolGenerator, you could merge straight away.

[dellaert]

Cool!