make greta depend on R 3.1

[njtierney]

This avoids this issue that came into place in 4.0.3

The LINPACK argument to chol.default(), chol2inv(), solve.default() and svd() has been defunct since R 3.1.0. Using it now gives a warning which will become an error in R 4.1.0.

[njtierney]

Also need to not pass through the LINPACK argument to chol.default and co

[njtierney]

test in test_functions.R returns:

══ Testing test_functions.R ════════════════════════════════════════════════════
[ FAIL 0 | WARN 1 | SKIP 0 | PASS 454 ]

── Warning (test_functions.R:113:3): matrix functions work as expected ─────────
the LINPACK argument has been defunct since R 3.1.0
Backtrace:
 1. check_op(chol2inv, c) test_functions.R:113:2
 2. run_r_op(op, a, b, other_args) helpers.R:187:2
 5. greta::chol2inv.default(...) /Users/njtierney/github/njtierney/greta-dev/greta/R/functions.R:411:2
 6. base::chol2inv(x = x, size = size, LINPACK = LINPACK) /Users/njtierney/github/njtierney/greta-dev/greta/R/functions.R:416:2

[ FAIL 0 | WARN 1 | SKIP 0 | PASS 454 ]

[njtierney]

Now provides:

==> Testing R file using 'testthat'

ℹ Loading greta

══ Testing test_functions.R ════════════════════════════════════════════════════
[ FAIL 0 | WARN 0 | SKIP 0 | PASS 453 ] Done!

Test complete