Pass args via ... to mvtnorm::pmvnorm()

[jdblischak]

To aid our discussion this afternoon. If we want to enable passing parameters to mvtnorm::pmvnorm(), this PR fixes that behavior. If we decide to keep this functionality, it would be nice to add some tests of realistic use cases so that this can be caught in the future.

xref: #235 (comment)

Note that the call to pmvnorm_combo() in gs_bound() does pass the ...:

gsDesign2/R/utility_combo.R

Lines 481 to 486 in f4a9e8d

	p <- pmvnorm_combo(lower_bound,
	upper_bound,
	group = analysis[k_ind],
	mean = .theta[k_ind],
	corr = corr[k_ind, k_ind], ...
	)

What this PR does is add the ... to the 2 calls in gs_prob_combo()

[jdblischak]

Note that the CI failures are due to a problem with installing the dependency {withr}

[nanxstats]

I approve this approach. Looking forward to those new tests! ✨