Add a deconvolve() method to mapper classes, which takes an alm array and removes the convolution kernel of the mapper.

This change enables deconvolution at the point of alm creation, where the mapper is available. The transform_maps() function has gained a deconvolve=True parameter, which deconvolves the resulting alms by default.

Functions for dealing with the convolution at later stages (depixelate_cls(), pixelate_mms_healpix()) have been removed accordingly. This makes computation of mixing matrices much more straightforward.

Closes: #87