Look over random field generator code

[fcooper8472]

The recent coverage failures appear genuine.

The method OffLatticeRandomFieldGenerator::SampleRandomField was not being called directly in a test, and on further inspection rand() was being called:

Chaste/mesh/src/random_field/OffLatticeRandomFieldGenerator.cpp

Line 104 in b98110e

return this->SampleRandomFieldAtTime(rNodes, rand());

A double was expected, and this caused that function to return nonsense.

I am putting in a temporary fix as part of #233, but I think the code needs properly looking over.

1. It is currently only tested in a "same as yesterday" manner, and it would be useful to verify some statistical properties of the noise (addressed in f23ec4a; see Utilise larger GitHub hosted runners #233)
2. The structure std::array<double, 3> is used as a corner point, and it may be more idiomatic to use a ChastePoint
3. I would like to understand how the time lengthscale interacts with the space lengthscale
4. Periodicity functionality is implied, but doesn't appear to exist.