Starting a GlobalFields class

Will provide a way to gather and scatter entire fields onto a single processor.
Uses include interfacing to other codes, and maybe parallel I/O.
Adding test case for GlobalField classes

Will check the gather/scatter facility
More updates to GlobalField classes

Not yet complete, but close...
GlobalField2D working

2D gather and scatter of data working. Some typo in 3D version,
so not yet working
Fixed GlobalField3D::msg_len

Forgot to add zsize when copying from 2D version
User manual section on GlobalField

Brief description of using GlobalField variables
Adding GlobalField classes

Gather / Scatter operations on entire mesh. Can be used for
coupling to external codes, may also be useful for I/O from a single
processor.

Doesn't affect any other code, so adding directly to master.