Fix variable bindings in use of dplyr and ggplot2 functions

[pcarbo]

@kevinlkx At several points you use dplyr and ggplot2 functions that produce "no visible binding for global variable" errors in the R package checks. These errors can be fixed following the instructions given here.

I believe this approach should fix all or most of these errors:

anno_finemap_res: no visible binding for global variable 'start'
anno_finemap_res: no visible binding for global variable 'end'
anno_finemap_res: no visible binding for global variable 'chrom'
anno_finemap_res: no visible binding for global variable 'id'
anno_finemap_res: no visible binding for global variable 'susie_pip'
load_weights : <anonymous>: no visible binding for global variable 'cm'
make_locusplot: no visible binding for global variable 'pos'
make_locusplot: no visible binding for global variable 'p'
make_locusplot: no visible binding for global variable 'type'
make_locusplot: no visible binding for global variable 'object_type'
make_locusplot: no visible binding for global variable 'r2_levels'
make_locusplot: no visible binding for global variable 'label'
make_locusplot: no visible binding for global variable 'susie_pip'
make_locusplot: no visible binding for global variable 'id'
summarize_param: no visible binding for global variable 'niter'
summarize_param: no visible binding for global variable 'value'
summarize_param: no visible binding for global variable 'group'

[kevinlkx]

fixed. Thanks Peter!