Add strict mode to vec merge, minor improvements and bug fixes

[m-mohr]

Stacked on #58 and #56 and #55.

Adds a strict mode to vec merge on top of the review fixes in #58.

• Breaking: vec merge is strict by default: problems that make the merged dataset invalid are errors. --no-strict warns and writes the dataset anyway. The README lists what each mode does per problem. merge_parquet gets an explicit strict (default True).
• Both engines report the same problems: missing required values (per collection), repeating ids, empty geometries, undeterminable collections, required properties removed by -i/-e, differing required collection-only properties.
• Constants that don't fit their schema type are reported with property and file in both engines; with --no-strict they are left empty instead of failing later in the writer.
• -i/-e also catch required properties that only a custom schema requires.
• With --no-strict, required columns with nulls are written as nullable instead of failing.
• Missing required values are reported per property with counts.

Extract from the Readme:

By default, vec merge is strict: problems that make the merged dataset invalid are errors
and no dataset is written.
With --no-strict, vec merge is fail-safe: these problems are reported as warnings and
the dataset is written anyway. Check it with vec validate afterwards.

Problem	Strict (default)	--no-strict
A required property has no value for some features	Error	Warning, the property is written as nullable
An id repeats within a collection	Error	Warning
The collection of a feature can't be determined	Error	Warning, the collection is left empty
-i or -e removes a required property	Error	Warning
A required collection-only property differs between the datasets	Error	Warning, the property is removed
An optional collection-only property differs between the datasets	Warning, the property is removed	Warning, the property is removed
A feature has an empty or missing geometry	Error	Warning, the feature is kept
A collection-level value doesn't fit the data type of its schema	Error	Warning, the value is left empty
A collection implements multiple versions of a schema, e.g. of an extension	Error	Error
The schemas of the datasets conflict	Error	Error

The strict mode only checks what a merge can break or check with little effort.
It doesn't validate the values against the schemas, e.g. patterns or value ranges,
so a merged dataset is only valid if the values of the source datasets are valid.

[ivorbosloper]

LGTM, but co-pilot has more remarks

[m-mohr]

Solved the remarks.