Update PETSc requirement to PETSc next

MatCopy_Nest has not made it to master yet, but we need it for form
assembly caching.
Missed one occurrence of s/master/next/
And update install.sh
Remove obsolete find_op2 module
Docs: properly rebuild API docs in livereload mode
Add optional argument headers to Kernel

Allows passing additional system headers to be included.
Allow executing user code snippet in kernel wrapper

Add an optional argument user_code to the Kernel constructor, which
allows specifying a code snippet which is to be executed at the very
start of the generated kernel wrapper. This allows executing some
code as a one-off, such as setting a random seed.
Document Kernel parameters
Add example usage of headers, user_code to docstring