create is_preperiodic function for points of projective space

[bhutz]

The function will take a point and map as input and return a boolean of whether or not the point is preperiodic for the map. Perhaps there should be an option to also return the period if the point is preperiodic

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​f2f6414

Merge branch 'master' into ticket/17855

[bhutz]

This incorporated a few additional changes

• there was an error in the error_bound canonical height computations which is now corrected

• heights and canonical heights are now computable for QQbar points

• methods to change QQbar points and maps to number field points and maps

• change_ring now accepts an embedding for points and maps

[gjorgenson]

These examples don't seem to work:

P.<x,y,z> = ProjectiveSpace(QQbar,2)
H = Hom(P,P)
f = H([x^2,y^2,z^2])
Q = P([1,1,1])
Q.is_preperiodic(f)

P.<x,y,z> = ProjectiveSpace(QQbar,2)
H = Hom(P,P)
f = H([x^2,y^2,z^2])
Q = P([QQbar(I),1,0])
Q.is_preperiodic(f)

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​06f4c35

17855: fix issues from review

[bhutz]

Yes, there were a couple issues with getting the right compositum base field. These should be fixed now.

[gjorgenson]

From the review, for is_preperiodic need:

• check that map is a morphism
• check that given point is in the domain of the map
• check that if over a number field, it is an absolute number field

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​476cf36

17855: fix additional input cases

[gjorgenson]

Everything seems to be working well and the changes fixed all of the issues from the reviews.