transforms in mcmc/api.py

[ludkinm]

When trying to use a custom MCMC kernel, inherited from MCMCKernel, the MCMC api raises an error at line 398 since self.transforms is None.
This is because the if block starting at line 387 always fails for any self.kernel which is not HMC or NUTS.

I propose removing the check that self.kernel is an instance of HMC or NUTS to allow for custom kernels.

This raises a problem: self.kernel.transforms may not exist for the check on line 388.
I propose changing that to first check that hasattr(self.kernel, 'transforms')

[fehiepsi]

Thanks for the fix and clearly explaining the issue!