Validate signal and background integral values in plot_DetPlot()

[mcol]

By enforcing that signal.integral.max > signal.integral.min we also avoid that the computation of the number of channels produces Inf (fixes #203).

Locally, running coverage on this produces this error from the test that uses multicore = 1:

Error in `checkForRemoteErrors(val)`: one node produced an error:
 could not find function ".validate_positive_scalar"

That error would go away if I used Luminescence:::.validate_positive_scalar, but that seems to be against the CRAN policy. Alternatively, one could move the validation checks before the self-call.

But in the meantime, let's see if CI is happy with it as it is.

[mcol]

Ok, this is fine! I'm not sure what's going on in my session (also a restart didn't help), but we can go ahead with this.

[RLumSK]

@mcol What is against the CRAN policy is the call with ::: but this is only needed for the checks locally. When the package is build the function has access to the namespace after access outside with :::.