allow non-default BPPARAM

[allonw]

To allow a user to limit scone's core usage in multicore or to register a cluster

Add BPPARAM=my_BPPARAM to every use of bplapply etc. and add my_BPPARAM=bpparam() to the function definition as recommended in biocparallel

[drisso]

This is now implemented in develop (commit cdff261).
By default, i.e., if no bpparam is specified, it uses the value of registered().