* Wrote abs_rounding function.
* Replaced part of the code in to_str with a call to round_digits.
* Added tests for different kinds of rounding in test_convert.py

* Add new symbol, 'M', for rounding to nearest with tying away from zero.
* Replaced part of the code in format_mpf with a call to abs_rounding
* Added tests for the M rounding mode.

* Add new symbol, 'Y', for rounding away from zero.
* Added tests for the Y rounding mode.

* The default for the rounding parameter is None.
* If rounding is None, a DeprecationWarning is raised; the rounding part of the code executes legacy code
* Else, the new round_digits function is called

* Made all the dunder methods that call to_str to include the rounding
parameter obtained from context. This is to avoid the new deprecation warning
in to_str.

Modified the call to to_str in mpc_to_str to include the rounding parameter
taken from context.