Core Objects

Now, here are some details about the core :
* The most important thing is that mathmaker is not a software intended to compute mathematical stuff, but to display it. This is the reason why the core is quite complex : the human rules we use to write maths are full of exceptions and strange details we don't notice usually because we're familiar to them. To re-create these writing rules on a computer is not really simple !
* So, all objects of the core are mathematical objects which can be represented, either as figures or as mathematical formulas
* Hence, all objects are not conceived according to their mathematical properties first, but according to their "displaying rules"
* base.py contains the most abstract classes :
- Copiable, mother class of all objects. Provides a deep_copy() method which ensures to create a new object completely identical to the given one
- Drawable, mother class of all geometrical objects. Provides the into_pic() method, which creates a picture of the object. All geometrical objects must implement a into_euk() method, which will create the eukleides file (.euk) to draw them. This method is used by Drawable.into_pic() to create the picture.
- Printable, mother class of all calculus objects. They all must implement the into_str() method, which tells how to display them (independently from the output's format). Note that Drawable was thought to be a Printable too, so that geometrical objects would have to implement into_str() as well. But this feature does not seem to be useful. So probably Drawable will only inherit from Copiable.
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I should certainly also update the documentation (I use doxygen), this would be of help...