fix: cast values, rename value.OptionalValue

[rot1024]

• When exporting a scene, the values of some properties linked to the dataset would become null. This has been fixed.
• pkg/value/OptionalValue has been renamed to Optional.
• pkg/value/Value.Cast method has been added to allow type conversion of values. Also, some type conversions such as string to URL and URL to string are newly supported.

[codecov]

Codecov Report

Merging #93 (b79260a) into main (1b99c6d) will increase coverage by 0.36%.
The diff coverage is 72.54%.

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   32.25%   32.61%   +0.36%     
==========================================
  Files         292      292              
  Lines       18812    18882      +70     
==========================================
+ Hits         6067     6159      +92     
+ Misses      11933    11915      -18     
+ Partials      812      808       -4     

Impacted Files	Coverage Δ
pkg/dataset/value.go	13.40% <0.00%> (-1.38%)	⬇️
pkg/value/latlngheight.go	21.42% <0.00%> (-4.66%)	⬇️
pkg/value/url.go	0.00% <0.00%> (ø)
pkg/value/latlng.go	33.33% <25.00%> (+6.06%)	⬆️
pkg/property/value.go	35.05% <92.30%> (+20.60%)	⬆️
pkg/dataset/value_optional.go	88.37% <100.00%> (+1.88%)	⬆️
pkg/property/merged.go	78.10% <100.00%> (-0.63%)	⬇️
pkg/property/value_dataset.go	100.00% <100.00%> (ø)
pkg/property/value_optional.go	95.34% <100.00%> (+0.75%)	⬆️
pkg/value/bool.go	66.66% <100.00%> (+66.66%)	⬆️
... and 9 more