Use preferred coordinate type for Path except when explicitly provided

[gpeairs]

Close #93. This is a behavior change for Paths where units were implicitly provided as the coordinate type of p0. For example, Path(0mm, 0mm) is now always the same as Path(). To set the unit type explicitly, use Path(mm, ...).

This makes it slightly less convenient to explicitly specify coordinate type units, and may introduce unnecessary conversions where p0 was used to ensure path units agreed with other geometry units. However, it avoids the much more common case of inadvertently specifying units in order to set p0, similarly causing unnecessary conversions down the line.

This adds a new constructor for Path (unit + point, e.g. Path(nm, Point(1,2)mm)) so will require a minor version bump.

[codecov]

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[gpeairs]

Holding this until #147 is merged, changing coordinate types is a risk for Booleans on mixed collections without the interface relaxation there.