ENH: update default precision for timer and enable setting precision globally

We need to adjust the default precision to be 4 floating point digits and figure out a way to set the float precisions for a lecture globally. - [ ] update QuantEcon.py with higher precision by default 0.4f - [ ] can we set the precision globally in a lecture through a variable in QuantEcon.py such as `qe.timings.float_precision(4)`