add internal argument to bypass repeated calls to `usable_data`

[AlexChristensen]

usable_data was added across all functions that handle raw data to ensure only numeric data are used in the analysis

this process includes coercing factors into numeric data

although this process is necessary, it gets called repeatedly across functions (e.g., bootEGA --> EGA --> EGA.estimate --> auto.correlate --> polychoric.matrix)

a simple internal argument such as skip_usable could be added to only perform usable_data at the top-level function (e.g., bootEGA) and skip all other usable_data checks in the subsequent functions (thereby eliminating redundant calls)

[AlexChristensen]

Resolution: an extra helper function needs_usable has been added and is used in checks for skipping usable data checks