Implement a function to map from cds-relative to tx-relative coordinates

[jorainer]

variant information is in most instances provided as cds-relative information. It would thus be helpful to have functions to map between tx-relative and cds-relative coordinates.

• return negative start coordinates if tx can not be found.
• return negative start coordinates if tx is not coding.
• return negative start coordinates if coordinate is within UTR.

[jorainer]

transcriptToCds: function returns start -1 and end -1 if:

• ID found?
• non-coding transcript(s)?
• start coordinate outside CDS?
• end coordinate outside CDS?

[jorainer]

cdsToTranscript:

• ID found?
• non-coding transcript?
• provided end < cds length?