Obscure error for passing EulerHeun instead of EulerHeun()

Hi,

I've wasted some time because I made a small mistake, and the error was quite obscure.
I passed EulerHeun instead of EulerHeun() as the solver argument.
Minimal example:

import jax.random as jr
from diffrax import diffeqsolve, ControlTerm, EulerHeun, MultiTerm, ODETerm, SaveAt, VirtualBrownianTree

t0, t1 = 0, 3
drift = lambda t, y, args: -y
diffusion = lambda t, y, args: 0.1 * t
brownian_motion = VirtualBrownianTree(t0, t1, tol=1e-3, shape=(), key=jr.PRNGKey(0))
terms = MultiTerm(ODETerm(drift), ControlTerm(diffusion, brownian_motion))
solver = EulerHeun
saveat = SaveAt(dense=True)

sol = diffeqsolve(terms, solver, t0, t1, dt0=0.05, y0=1.0, saveat=saveat)
print(sol.evaluate(1.1))  # DeviceArray(0.89436394)

This returns

--> 137 assert type(term_contr_kwargs) is tuple

For the Euler solver, the error is different:

TypeError: functools.partial() argument after ** must be a mapping, not property

Maybe this can be improved? I can take a look later if I have some time.

Start at the diffeqsolve entry point with the solver argument, comparing the EulerHeun and Euler examples when the solver class is passed instead of an instance. Reproduce the reported errors and add coverage showing that this misuse produces a clearer error message.