Allow spllicing of SP3 files

A recurrent demand on the forum is to be able to use several consecutive SP3 files. One suggestion made was to use an AggregateBoundedPropagator with a list of BoundedPropagator retrieved from each SP3 file. This works great when the file producer takes care to have the last point of one file at the same date as the first point of the next file (typically at midnight), knowing that there may be a discontinuity as the files are produced on different batches of measurements. This however does not work when there is a gap between the last point of one file and the first point of another file. As an example, some agencies produce files covering 1 day at 5 minutes interval, and these files start at 00:00 and end at 23:55. Using AggregateBoundedPropagator, the 5 minutes gap between the end of one propagator and the start of the next propagator cannot be filled: an exception is raised if a computation is attempted during the gap.

In order to alleviate this problem, it would be nice to be able to splice SP3 files together, taking care to drop one point if both files provide data at exactly the splice point. In this case, dropping the file from the earliest file and keeping the file from the newest one seems fair.

For consistency, splicing should be allowed only for files with matching metadata (same time system, same list of satellites, same interval between epochs…).