Merge with chaste-codegen

[kwabenantim]

Summary
Merge chaste-codegen-sbml into one package with chaste-codegen.

Advantages include:

• Simpler to install during configuration step in Chaste builds.
• Easier to synch dependency versions between the two packages e.g. Jinja.
• Good timing to create a structure which we can later extend to add support for generating Chaste code from other formats.

Disadvantages include:

• Chaste-codegen is already published and in use so changes should minimize impact on current users and existing references.

[kwabenantim]

The following structure has been proposed (by @fcooper8472):

chaste-codegen (new super-package)
|
|---chaste-codegen-cellml (current chaste-codegen)
|
\---chaste-codegen-sbml