Introduce an instrumentation_level parameter

Instead of the current timer option, there should be a parameter instrumentation_level parameter which allows the following values:

• 0 (default) no instrumentation
• 1 The program as a whole is measured
• 2 A whole operator application or residual/jacobian assembly is timed
• 3 The assembly methods are timed separately
• 4 Parts of kernels, such as SF stage 1..3 are timed separately