Modify behaviour of file backup option for solvers, NekMesh and FieldConvert

The option NEKTAR_DISABLE_BACKUPS in CMake is no longer used for enabling or disabling the backups.

The alternative approach proposed in this branch is to disable the writing the backups in one of two ways:

• a command line option --forceoutput to be consistent with the existing option inside FieldConvert.

• setting an environment variable (NEKTAR_DISABLE_BACKUPS) to overwrite the backups' behaviour. The user could use the following command in the terminal to disable the backups:

  $ export NEKTAR_DISABLE_BACKUPS=1

While this environment variable exists, it will prevent backups from being written.

Closes #209 (closed).