OverlayNG: Interpolate M values

[dbaston]

This PR adds support for interpolation of M values in OverlayNG. M values from the input are preserved and M values at constructed points are populated via linear interpolation if possible. Unlike the Z dimension, the ElevationModel is not used to populate the M value of output coordinates that do not intersect segments with M values.

This does not cover all cases - for example, if snap-rounding is required, I expect that M values will be lost. I will work on adding test cases so that these usages can be covered.

By not assuming the use of the Coordinate type, these changes also move further in the direction of using XY inputs without carrying around a Z value.

[mwtoews]

Also web/content/usage/download.md will need to be updated to mention "C++14 compiler". There are several other instances, find them with:

 git grep -i "c++11"

[dbaston]

@pramsey @dr-jts Are you comfortable with the semantics used for M values here? ("Interpolate like Z but don't use the ElevationModel")

[dr-jts]

Are you comfortable with the semantics used for M values here? ("Interpolate like Z but don't use the ElevationModel")

Yes, seems reasonable.

[dbaston]

Performance is looking OK for the XY(Z) cases. Maybe a small regression in intersection of inputs that are already fully noded (triangulation results, world.wkt).