Issue #259 Make pyproj an optional dependency when parsing CRS strin…

[JohanKJSchreurs]

Issue #259 continued: Make pyproj an optional dependency when parsing CRS string / EPSG code

• Now the import of pyproj happens locally in the function that needs it.
• CRS values are only processed further with pyproj when they are neither type int nor a string that represents an integer.

[soxofaan]

thanks,
merged already to get builds working again

(If necessary, I'll do some follow-ups directly on master)

[soxofaan]

did some refactoring in the end in b2a0e00 (tagged this ticket wrong apparently).
Biggest change is that the normalization function also can return a WKT2 CRS string (not only EPSG int), because that is also allowed in openEO API. This made it actually possible to simplify the function and rework the optional pyproj import stuff

[soxofaan]

pff that pyproj stuff is highly version sensitive. It took more commits that I anticipated to get things right. I should have worked in a feature branch in hindsight 😛