Collisions tests identical to test le_grids

[Joseph Parker]

The two collisions tests are identical to each other, and moreover, both are the same as the le_grids test.

Is there an easy way to modify these into useful tests of collisions? Perhaps this is something @daviddickinson or @adrianjhpc might have thought about?

Otherwise, it's probably better to just delete them.

[Joseph Parker]

Delete collisions tests, which are a repeat of le_grids tests

Addresses #30

→ <<cset 1b046dfa7c28>>

[David Dickinson]

I'm unfortunately not very familiar with the tests but I did notice these tests seem to have collision_model="none" so certainly aren't testing the collisions! I'm not sure there's an easy way to unit test the collision operators currently but it might be nice to have some slightly bigger tests that check the conservation properties.

[David Dickinson]

Collisions test removed.