Implement hcubature_count and hcubature_print

[mikeingold]

Changes

• Implements an hcubature_count function, as suggested by hcubature_count #68, which counts the number of integrand function evaluations and returns (I, E, count).
• Implements an hcubature_print function, shamelessly adapted from QuadGK.quadgk_print (credit to @stevengj), which prints a line f($x) = $(f(x)) for every integrand function evaluation and returns (I, E, count).

[mikeingold]

I added another test for the hcubature_print method without an io arg to address test coverage. This test suppresses stdout for that particular call to avoid cluttering the test logs.