Suggestion: set default of maxP to NULL

Current biobank and meta-GWAS are well-powered enough that almost all will contain a P-value smaller than 1e-14, so there isn't a single value that is sensible default for truncating the Y axis. This PR sets maxP=NULL as the default.

Maybe also this parameter could have a different name since the value it accepts is interpreted as -log(P) and would also depend on the value of the logp argument.