Add keyword prec for exponential and logarithm of Drinfeld modules

[xcaruso]

Currently, the methods exponential and logarithm for Drinfeld modules return a LazyPowerSeriesRing for which not all methods are available.
We add a keyword prec. When it is Infinity (which is the default), the behavior is unchanged. However, when we pass in a finite number, a classical power series (up to the given precision) is returned.

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[mantepse]

Off topic: Which methods are missing in LazyPowerSeries?

[xcaruso]

e.g. sqrt (it is implemented only for series congruent to 1 modulo x), nth_root, continued_fraction...
Also printing more terms is a little bit tricky; we need to set the parameter parent.options.display_length (which moreover is global, we cannot easily change this for just one shot).

[mantepse]

e.g. sqrt (it is implemented only for series congruent to 1 modulo x), nth_root,

Oh, I missed the possibility of taking out x^n!

continued_fraction...

I actually have code for that, but was too lazy to get it in.

Also printing more terms is a little bit tricky; we need to set the parameter parent.options.display_length (which moreover is global, we cannot easily change this for just one shot).

I agree. Maybe we can discuss this at sd128?