GSoC Week 2

This week was focused on fixing underlying errors in low-level leading term methods, which were popping up while making amendments to the limits code.

I encountered 3 non-terminating limits and several other wrong answers. After some discussion with my mentors, it was decided that the _eval_as_leading_term methods should have a logx parameter. I added these to all methods.

Apart from this, log._eval_as_leading_term method should raise when the argument is a singularity (0 or complex infinity) of log, and this was added. Previously, it would just return log(arg) (the original expression whose leading term we were trying to find).

There were a few other miscellaneous issues with other functions, which can be found on my PR. Currently, 4 test failures remain, so this work should finish up within 1-2 days, after which only review changes would be required. I plan on tackling the issue of where to call aseries in the series code.