General Improvements and clarifications

[davidrohr]

• Improve text a lot
• Remove obsolete parts and figures.
• Relax requirements for level 1, such that 40kw are enough in any case.
• Rewrite requirements for level 2 and disentangle the core phase and full run measurements with respect to the 10 power averaged measurements.
• Add accuracy requirements to the 4th aspect.
• Clarify that estimating power means substituting by upper bound.
• A measurement that qualifies for a higher level in one aspect automatically also qualifies for a lower level.
• Change accuracy requirements of power meters as discussed with Natalie and Paul.
• Rewrite and clarify the text about power measurement samples.
• Add a requirement on the maximum interval between power measurements reported by the power meter (should be short compared to the core phase).

Now include pull request 83 "Various improvements of the text". This was rebased onto here, so it can be merged.

Fixes #81 Fixes #80 Fixes #77 Fixes #75 Fixes #74 Fixes #65 Fixes #63 Fixes #62 Fixes #56 Fixes #82 Fixes #66