Trac: Allow non-oxmeta annotations for variables

[MichaelClerx]

https://chaste.cs.ox.ac.uk/trac/ticket/1903

Child of https://chaste.cs.ox.ac.uk/trac/ticket/1873. Additional related tasks in https://chaste.cs.ox.ac.uk/trac/ticket/2529.

[MichaelClerx]

Think this breaks down as:

• [x] Allow annotations from other ontolgies #141
• [ ] Stop using local names in generated model code
• [ ] Stop using local names in e.g. sim:membrane_voltage

[jonc125]

The remaining tasks here are just a duplicate of #169.