Remove "const" modifier from Cython file

Old Cython versions don't support the const keyword.
Add Model.get_sequence_names()
Replace generator expression by list comprehension

Old Cython version doesn't support generator expression.
Move C level imports to separate declaration file

This forces the C symbols to be accessed more explicitly. Furthermore,
it will hopefully fix two warnings on the test server.
Fix return type declaration of table_get_header
Fix some spaces
Use functions for decoding/encoding
Use more compact combined declaration+assignment
Declare some internal variables as C-types

No need to cast them to python objects in between calculations.
Merge branch 'active-sequence' into memaccess_elements

Conflicts:
	src/cern/cpymad/model.py
Return list of element dicts from get_elements