Add RClone to suggests

[zkamvar]

I got a message from Kurt Hornik this morning:

These seem to have undeclared package dependencies in their unit test
code (R files in tests subdirs), see below.

Can you pls fix as necessary? (Add the missing package dependencies to
Suggests, I guess.)

Please note that these issues are currently not yet detected by the CRAN
incoming (or regular) checks.

$poppr
'::' or ':::' import not declared from: ‘RClone’
'data(package=)' call not declared from: ‘RClone’

This refers to a check that happens only if RClone is installed and I believe it doesn't even occur on CRAN, but I will add it to suggests because it doesn't really cost me anything.