Allow to pass exponential remainder to functions that compute it

[schillic]

If we call several functions that compute _expm_remainder (like in Reachability), it would be better to instead compute it only once and pass the result around.

[schillic]

We also have two functions to compute the remainder: _exp_remainder and _exp_remainder_series. Currently we always use the first one. This function could be the parameter to compute the remainder, so remainder=_exp_remainder by default and then one can pass the remainder matrix or the alternative algorithm _exp_remainder_series if desired.