Performance improvements and collect test improvements

[ZedThree]

Rebase of #101 to account for the moved source files

Fixes #68

Refactor of collect tests to make them run in a reasonable amount of time:

• Cache dump files for each grid scenario
  • This gives a 25-30% speed-up
• Remove some combinations of test parameters
  • The slicing test alone was almost 30% of the runtime due to the number of combinations
  • Reduced total number of tests from ~600 to ~200
• Only make one of each kind of field in the test dump files
  • ~50% of the runtime of the tests was solely due to datafile.open, which we have to do at least once for every call to collect, sometimes more than 50 times
• Reduce duplicated code by parameterising over the grid scenarios

Overall this reduces the time the tests take from >35 minutes to <2

[mikekryjak]

This is very impressive!