Please support multiple frames in PGM file format

The PGM file format (http://netpbm.sourceforge.net/doc/pgm.html) allows for multiple frames in one file, called a sequence.
The frames are just written, without gaps or padding, sequentially to the file.
The number of frames in the file can be determined by the (fileSize - Header)/(size_of_one_frame)

If I open such a file in ImageJ, only the first frame is displayed.
Please improve the PGM file reader, so it can read multiple frames as a sequence, just like the RAW importer does.
[sequence_3_frames_38x38px_8bit.zip](https://github.com/imagej/imagej/files/5675211/sequence_3_frames_38x38px_8bit.zip)

As a work-around, you can import the file using the "RAW" importer. For example, for the attached file, use uint8, size: 38x38 px, 3 frames.

Tested on ImageJ 1.53e.

Start by locating the PGM file reader and comparing it with the RAW importer’s sequence handling. Use the attached sequence_3_frames_38x38px_8bit.zip, with its 38x38 8-bit three-frame data, to verify that the reader opens all frames as a sequence; done means the file no longer displays only its first frame.

Written by the indexing model from the issue text.