Add custom __repr__ for Kernel

[alexfikl]

Before:

LaplaceKernel(
	dim=2,
	expression=Call(Variable('log'), (Call(Variable('sqrt'), (Sum((Product((..., ...)), Product((..., ...)))),)),)),
	global_scaling_const=Quotient(1, Product((-2, Variable('pi'))))
)

After:

LaplaceKernel(dim=2, expression=log(sqrt(d[0]*d[0] + d[1]*d[1])), global_scaling_const=1 / ((-2)*pi))

Thoughts? I mostly added it because the repr for the expression is quite unhelpful in the current version.

[inducer]

I agree that seems better.