#### Reference Issues/PRs
Closes: #1428 

#### What does this implement or fix?
Create an empty-index type. This required change in the Python and in
the C++ layer.
* In the C++ layer an new index type was added.
(IndexDescriptor::EMPTY). It does not allocate a filed in the storage
(similar to how row range index does not allocate a field). The checks
for index compatibility are relaxed, the empty index is compatible with
all other index types and it gets overridden the first time a non-empty
index is written (either through update or append). On write we check if
the dataframe contains 0 rows and if so it gets assigned an empty index.
* The logic in the python layer is dodgy and needs discussion. In the
current state the normalization metadata and the index descriptor are
stored separately. There is one proto message describing both DateTime
index and Ranged Index. The current change made it so that in case of 0
rows the python layer passes RowRange index to the C++ layer which
checks if there are any rows in the DF. If there are rows Row range
index is used, otherwise empty index is used. Note the
`is_not_range_index` proto field. IMO it needs some refactoring in
further PRs. It's used in the python layer to check if the first column
is index or not.
#### Any other comments?

Merge this after: #1436

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---------

Co-authored-by: Vasil Pashov <vasil.pashov@man.com>

Refactor index.hpp

Move the functions in a .cpp file. Add explicit template instantiations
for the CRTP base class in the cpp file.

#### Checklist

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   Checklist for code changes...
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---------

Co-authored-by: Vasil Pashov <vasil.pashov@man.com>

…th dynamic schema

…xed dfs with dynamic schema