Change default dim in DTMDensity for high dimension

[mglisse]

In DTMDensity (and Tomato), when the points are in "high" dimension, we recommend using a small value for dim instead of the default: the ambient dimension. But if we recommend it, why not make it the default behavior? Something like

• if dim=None and the deduction would choose a value larger than 10 (?), then just pick 10 (or even go down to 2 while we are at it?) and print a warning advertising the next point
• if dim is specified by the user, they can use any value they like

DTMDensity (without normalization) ends with density = dtm ** (-dim / q). This exponentiation is pretty useless if we are only going to use it in a log in Tomato's logDTM. Without it, only q needs to be small enough, dim becomes unused, and the behavior matches Tomato's doc before #1074.