Add is_header_parsed flag to quickly check if header information is available in BaseRawIO

[h-mayorquin]

The BaseRawIO class has the method parse_header that extracts all the information from the formats and therefore is usually a computational heavy process:

https://github.com/h-mayorquin/python-neo/blob/e3742da276fe3bb9a1ea327489ebdbf06c026956/neo/rawio/baserawio.py#L166-L181

I propose adding a simple flag called is_header_parsed that is set to True after the header is parsed (i.e. parse_header is called). That way, users can check quickly if the header is alredy parsed without having to check for the contents of the header directly.

That is, we follow the mantra that "explicit is better than implicit" here from the Zen.

Use case in point:

From spikeinterface, we sometimes require one or two properties from the header before intializing the neo reader:

https://github.com/h-mayorquin/spikeinterface/blob/1b8ae2c0907aee49ca9cb44e19c78d9bbfadf6e2/src/spikeinterface/extractors/neoextractors/plexon.py#L62-L64

However, within the neo base extractor we then re-parse the whole file at initialization:
https://github.com/h-mayorquin/spikeinterface/blob/1b8ae2c0907aee49ca9cb44e19c78d9bbfadf6e2/src/spikeinterface/extractors/neoextractors/neobaseextractor.py#L40-L64

We could avoid this with an explicit flag such as the one proposed into the PR without having to make assumptions about what is on the header attribute.

@alejoe91 @weiglszonja

[samuelgarcia]

Good idea.
Thank you.

[weiglszonja]

Sounds good to me, thanks @h-mayorquin!