Introduce dataframe file type to replace / supplement beddb and bed2ddb

The goal of this PR is to discuss the introduction of a new file type that replaces the `beddb` and `bed2ddb` formats. This file type will be able to store any type of data and be used as backing for gene annotations, bed-like regions, arbitrary points, etc...

## Questions to address:

1. Tile API: the current API takes a zoom level, start and end position. It works right now because any genomic data is converted to a linearized representation where chromosomes are concatenated using a given chromosome order.

Dataframe-backed files will not have this limitation. The tile API will have to have a chromosome order associated with it to indicate which data should be retrieved between coordinates x0 and x1.

Example API:

```
def get_1D_tile_data(
filename='my_file.tsv',
tile_position=[1,0],
group_column=['chr'],
position_columns=['start', 'end'],
group_order=[('chr1', 1000), ('chr2', 5000), ('chrX', 4000), ('chrM', 3000)]
)
```

2. Column to use as the index: A dataframe may have the start and end positions at arbitrary positions. The request should include an indicator of which columns to use for the positions of the data.

## Use cases

1. Replacing the current beddb and tile bed2db formats.

## Perfomance

Filtering a 970K line file takes about 200ms. It may be possible to improve this through parallelization, sorting, indexing or subdividing the file into sections (e.g. chromosomes)