Refactor pga

[r0mainK]

OKay so a lot of commits, but most should not be hard to review/were already reviewed in the previous PR:

commit 1:

• Move the parser logic to pga/parser.go
• Rename the parser methods
• Move the formatting logic to pga/format.go
• Add float handling

commit 2

• Remove all of the dataset specific logic
• Create Repository interface: used to store and access data from rows of the CSV, as well as go back to CSV representation
• Create Filter type: no more FilterIndex bs, just a function takin a Repository and returning a boolean.
• Create Dataset interface: used to read header of the CSV, and apply given function on each row. Incorporates most of the logic from the old Index, but will allow handling of dataset specificity via it's methods, and CSV specificity via it's attribute (e.g. legacy handling)
• Create Datasets slice storing know datasets
• Create header errors

commit 3

• All the logic from pga.go specific to the siva dataset is contained here.
• Types for the interfaces: SivaRepository, SivaDataset with appropriate function
• Helper SivaRepositoryFromCSV used only in the ForEach method. I kept it separated because it's prettier this way.

commit 4

• Repository is an interface now, nothing new from the old PR

commit 5

• Dataset args handling, nothing new from the old PR

commit 6

• Move the setupContext function to cmd/context.go, because it is used in both list and get command, but was in the cmd/get.go file for I guess legacy reasons.

commit 7

• Add datasetName to getIndex, nothing new from the old PR

commit 8

• Add .index to indexName, fix typo, remove unused flag, nothing new from the old PR

commit 9

• Update the list command, basically just using the new interfaces.

commit 10

• Update the get command, basically just using the new interfaces.

commit 11

• Fix docstring of siva command
• Removed unnecessary else before return

commit 12

• Help was being printed on error, not anymore
• Command cancellation was ugly, not anymore

commit 13

• Use new chegaa/pb for less dependencies and LoC

[vmarkovtsev]

(except for foo binary - communicated on-site)