Replace `PD_AMORPHOUS` with `PD_AMORPHOUS_PEAK`

[rowlesmr]

will close #129

PD_AMORPHOUS was insufficient to describe the same amorphous phase being present in more that one diffractogram; it could only link to peaks and diffractograms. The new category gives amorphous peak positions and intensities, and links them to actual instansiations by PD_PEAK in diffractograms.

Essentially, an amorphous phase has peak positions and intensities, widths are an implementation detail, which is the same as how crystalline phases work.

[rowlesmr]

Is it possible to add a method to give point-wise contributions of an amorphous phase to a diffractogram?

[rowlesmr]

Is it possible to add a method to give point-wise contributions of an amorphous phase to a diffractogram?

Ah! Forgot... already possible through PD_CALC_COMPONENT.