Allow custom figure for subplot grid in axisgrid

[pas-calc]

Allow figure-level JointGrid plot to be defined on already-existing (sub)figure.

MWE:

fig=plt.figure()
subfigs=fig.subfigures(2) # 2 subfig rows
J = sns.JointGrid(data=df, x=x,y=y, f=subfigs[0]) # init JointGrid on subfigure
J.plot(sns.scatterplot, sns.boxplot) # plot: joint, marginals
ax=subfigs[1].subplots() # some other data
# ax.plot([1,2,3],[4,9,3]) # other data
plt.show()

we could also add this option to the convenience function sns.jointplot (extra kwargs are not passed to JointGrid init)
with a default None figure.

related
#399
#2059