better names for the fields of pseudo-metric spaces

[affeldt-aist]

analysis/theories/topology.v

Lines 4544 to 4547 in 0e392b5

	ax1 : forall x (e : R), 0 < e -> ball x e x ;
	ax2 : forall x y (e : R), ball x e y -> ball y e x ;
	ax3 : forall x y z e1 e2, ball x e1 y -> ball y e2 z -> ball x (e1 + e2) z;
	ax4 : entourage = entourage_ ball