Dynamic model improvements

Related to #142 (closed)

• Make cellml_converter.py compatible with Python > v.3.8 (xmltree.etree.getchildren() is discontinued)
• Add -I${CARP_DIR}/_build/physics/limpet to make_dynamic_model.sh to support CMake-based compilations
  • Not ideal, as the _build directory could be called differently depending on how the user invoked CMake
  • and will likely raise a directory not found warning for make users
  • but it will work for most people, I guess. Open to other suggestions if someone has an idea. Creating the ION_IF.h file in the source directory rather than the build directory with CMake? I do see the advantage of out-of-source builds but for limpet we put a lot of generated files in the source directory anyway (for example the ionic models).
• Support for atan(), sin() and floor() in .model to support importing the Heijman model from CellML (see Q&A)
• Better handling for e notation in CellML files (issue with the Heijman model as well)