Set the fluent version programmatically

[seanpearsonuk]

At the moment we support use of these environemnt variables:
PYFLUENT_FLUENT_ROOT
AWPROOT_NNN

We want to allow

import ansys.fluent.core as pf
pf.set_fluent_version(<path to fluent>)
pf.set_ansys_version(<path to ansys>)
pf.launch_fluent()

these settings would supersede the environment variable settings, which are still usable

[seanpearsonuk]

#702