(core) Lossless imports

Summary:
- Removed string parsing and some type guessing code from parse_data.py. That logic is now implicitly done by ValueGuesser by leaving the initial column type as Any. parse_data.py mostly comes into play when importing files (e.g. Excel) containing values that already have types, i.e. numbers and dates.
- 0s and 1s are treated as numbers instead of booleans to keep imports lossless.
- Removed dateguess.py and test_dateguess.py.
- Changed what `guessDateFormat` does when multiple date formats work equally well for the given data, in order to be consistent with the old dateguess.py.
- Columns containing numbers are now always imported as Numeric, never Int.
- Removed `NullIfEmptyParser` because it was interfering with the new system. Its purpose was to avoid pointlessly changing a column from Any to Text when no actual data was inserted. A different solution to that problem was already added to `_ensure_column_accepts_data` in the data engine in a recent related diff.

Test Plan:
- Added 2 `nbrowser/Importer2` tests.
- Updated various existing tests.
- Extended testing of `guessDateFormat`. Added `guessDateFormats` to show how ambiguous dates are handled internally.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3302

app/common/ValueParser.ts

@@ -36,18 +36,6 @@ export class ValueParser {
 class IdentityParser extends ValueParser {
 }
 
-/**
- * Same as basic Value parser, but will return null if a value is an empty string.
- */
-class NullIfEmptyParser extends ValueParser {
-  public cleanParse(value: string): any {
-    if (value === "") {
-      return null;
-    }
-    return super.cleanParse(value);
-  }
-}
-
 export class NumericParser extends ValueParser {
   private _parse: NumberParse;
 
@@ -225,7 +213,6 @@ export class ReferenceListParser extends ReferenceParser {
 }
 
 export const valueParserClasses: { [type: string]: typeof ValueParser } = {
-  Any: NullIfEmptyParser,
   Numeric: NumericParser,
   Int: NumericParser,
   Date: DateParser,