Refactor API, add Parser Interface

[suyashkumar]

This change refactors the core go-dicom API to use an interface called Parser. This allows for more elegant dependency injection of items (like channels, previous data elements) with a clean end user api (so that the user doesn't have to worry about passing in those references themselves). Using an interface like this will also make typical testing easier on the end user, and is also more canonical golang imho. This closes #11 and addresses #3. This also puts some plumbing in place for #5.

// Parser represents an entity that can read and parse DICOMs
type Parser interface {
	// Parse DICOM data
	Parse(options ParseOptions) (*DataSet, error)
	// ParseNext reads and parses the next element
	ParseNext(options ParseOptions) *Element

        // The below two should go away as we continue to refactor the library
	DecoderError() error // This should go away as we continue refactors
	Finish() error       // This should maybe go away as we continue refactors
}

Usage:

p, err := dicom.NewParserFromFile("myfile.dcm", nil)
opts := dicom.ParseOptions{DropPixelData: true}

element := p.ParseNext(opts) // parse and return the next dicom element
// or
dataset, err := p.Parse(opts) // parse whole dicom

[suyashkumar]

This function came over mostly as is for the refactor from ReadElement but this will itself need additional refactors to hopefully make it more modular.

[suyashkumar]

new interface! :)
(github keeps collapsing this file because it thinks it's too large of a diff)

[suyashkumar]

was thinking maybe dicom.Reader would be cool, but Parser is maybe more clear? would be nice to pair with dicom.Writer for the writing func in this lib

[suyashkumar]

for folks who just want to parse a file and not do any intermediate header checking we can likely make a helper function to collapse this

[suyashkumar]

From before when this was ReadElement it has not returned an error, rather it sets an error on the dicomio.Decoder...I expect this was done in the past because it is convenient to just have a *Element returned and deal with the errors later (particularly given there seem to be recursive calls to this function)...should see if it makes sense to have this return a canonical error though

[suyashkumar]

deal with this err

[suyashkumar]

deal with this err