Provides an interface to adjust nhdplusTools `memoise` cache.
Mode and timeout can also be set using environment variables. `NHDPLUSTOOLS_MEMOISE_CACHE` and `NHDPLUSTOOLS_MEMOISE_TIMEOUT` are used unless overriden with this function.
nhdplusTools_cache_settings(mode = NULL, timeout = NULL)
character 'memory' or 'filesystem'
numeric number of seconds until caches invalidate
list containing settings at time of calling. If inputs are NULL, current settings. If settings are altered, previous setting values.