Minor MPI corrections

[bevanwsjones]

Small fixes need to be made after the current merger of #79.

• reduceAll should be allReduce.
• The mpi environment should be parsed to functions, not the members of the class.
• Update of documentation if required from the above

[bevanwsjones]

See below comment, clean up environment.

https://github.com/exasim-project/NeoFOAM/pull/79#discussion_r1662348915

[bevanwsjones]

Add docs to basic page.