Use CppyyKernel to run the notebooks instead of Xeus-cling.

The notebooks have been modified to go along this change; I tried as much as possible not to introduce changes in the actual content of the notebooks (minus some stylistic changes: initialization through braces replaced some initialization by parens, as the braces require the cppyy.cppdef magic the change seemed organic enough).

The main changes are:

• Many references to Coliru have been suppressed and replaced with code run in a cell with clang magic.
• Some cppyy related magics have been added for specific cells that require it.
• The Docker image has been updated to reflect this.
• Binder has been completely modified.
• README and comments have been updated to suppress references to Xeus-cling and provide up-to-date information regarding installation.

Closes #101 (closed).