Provide capability to trim traces exactly to selection

[chad-earthscope]

Background: Currently the "selection" capability of the library limits the file reading functionality to miniSEED records that contain data for the selection. In most selections with a specified time window, this results in a bit more data, before and after, than is specified in the selection.

An enhancement could provide the capability to further trim the traces such that the data is limited to exactly what is specified in the selection.

One idea for this is to provide a new function that takes a trace group or list along with a selection and performs the trimming. A caller would first call one of the file reading routines with a selection and then call the trimming function with the same selection.