ENH: support extracting the M values as a separate attribute field (column)

[jorisvandenbossche]

See https://github.com/geopandas/geopandas/issues/2188

[brendan-ward]

If I understand M values correctly, these are like another dimension on the coordinate (i.e., in addition to Z), rather than always a single value per geometry? If so, how would we represent the M values within a column? As a list that is in coordinate order within the geometry (which then becomes invalid if the geometry is modified, e.g., normalized, simplified, etc)

[jorisvandenbossche]

Yes, indeed, for non-point geometries that will need to be a list of values. That doesn't make those results very easy to work with (since eg pandas doesn't provide much support to work with a column of lists). But I think my main idea for opening the issue was to give at least the possibility to access the values for the user (as right now with python tools, you cannot do this), and afterwards it is up to them to see what they can/want to do with it