Add disc setup

[dmentipl]

Need to replicate Phantom's setup_disc.F90 which allows the user to choose:

• what disc or discs to setup,
• round one or more stars, either as sink particles or an external potential,
• whether to add dust, as one-fluid or two-fluid, how to add it, how many dust species, i.e. grain sizes,
• whether to add planets,
• and so on.

Also need to write a "distribution" function to generate the particle positions and velocities given a radial surface density profile. This replicates Phantom's set_disc.F90.

[dmentipl]

Much of this is now implemented. We can set up a disc with arbitrary surface density profile, using the locally isothermal eos.

Still to do:

• set particle mass from Toomre Q, or something else
• add warps
• support for external forces, e.g. black hole stuff
• add correction for self-gravity
• calculate thermal energy for non-isothermal eos

[dmentipl]

Some related issues:

• equation of state for binary dics, i.e. ieos=6 and ieos=14
• estimating SS-alpha from <h>/H
• modify set_positions to account for dust-gas mixture
• (maybe) add reading Phantom .setup files