Resolve an issue with np.take and <U0 dtype

Fixes an issue caused by a bug in numpy internals on empty strings.

Fix multi-column import

Fix an issue with corrupt layout when the same column is being imported multiple times.

Weaker boolean infer in nim

Replaced tight boolean infer to tight boolean infer by ignoring capitalization.

Patch for join keyword "merge_keys"

Minor patch for keyword "merge_keys" in join.

See f176474 for details

Nim backend + other changes

Nim backend for csv importer.
Improve excel importer.
Improve slicing consistency.
Logical cores re-enabled on *nix based systems.
Filter is now type safe.
Added merge utility.
Various bugfixes.

Adding merge utility to Table and joins

This update features:

New:

• join with keyword merge_keys.
• Table.merge as a numpy accelerated helper for merging columns.

Fix:

• bugfix on excel readers deprecation warning.

Minor bug fix on pivot transpose

Pivot transpose now raises ValueError when duplicate column names are given as input.

Fix xlsx headers

Fix issue with xlsx headers

Environment file & Excel changes

Hidden excel rows/columns will be importer.
.env file can be created for development

Packaging fix

Fix wrong .so being published