Introduce Cactus parameters to disable using stress-energy tensor and MP jacobian

These Cactus parameters switch off the stress-energy tensor and the jacobian, improving performance if the code is compiled dynamically (e.g. OpenCL).
Add timing and operation counts to generated loops (but leave commented out)

Commented out; need to find efficient way to enable/disable this.
Take calc_every and calc_offset into account when selecting boundary conditions
Use const qualifier before type name, not after type name

E.g. change "int const" to "const int". Blue Gene/Q requires this for vector types.
Clean up several ATTRIBUTE_UNUSED and BUILTIN_UNREACHABLE
Add braces around case blocks

Otherwise one cannot declare variables there.
Add "default" statement to switch statements, aborting the code

This avoids compiler warnings since it tells the compiler that one case will match.
Improve vectorization optimizations
Remove outdated comment: IfThen works fine with vectorization
Do not convert integers to reals when not vectorizing
Add documentation for StandardUpwindDifferenceOperator
Optimize FD operators

Re-use another FD operator if it is the same:
- map D[2,1] to D[1,2]
- map D[3,3] to D[2,2], and transpose the input array descriptor

This reduces code size.
Simplify STORAGE statements: do not generate if-cascade any more
Change indentation of READS and WRITES statements in schedule.ccl
Handle *NoSync cases for READS and WRITES statements in schedule.ccl