Fixes #282: implement initializing using an expression
work both for element() and elementPtr()

here is an example:
```
auto u = Xh->element(sin(pi*Px()),"u");
auto v = Xh->elementPtr(sin(pi*Px()),"v");
// initialize through a ginac expression
auto w = Xh->element(expr( option(_name="functions.g").as<std::string>() ));
```
Update #282: example using new initialization from `element()`
Merge branch 'develop' of https://github.com/feelpp/feelpp into develop