Add support for SpiceRef associated reference frame

Add new .frame property to all SpiceRef objects:

>>> SpiceRef('Jupiter Barycenter').frame  # SpiceRef from toolkit
'IAU_JUPITER_BARYCENTER'

>>> SpiceRef('JUPITER').frame  # SpiceBody
'IAU_JUPITER'

>>> SpiceRef('JUICE').frame  # SpiceSpacecraft
'JUICE_SPACECRAFT'

>>> SpiceRef('JUICE_JANUS')  # SpiceInstrument
'JUICE_JANUS'

>>> SpiceRef('JUICE_JUP_AMALTHEA_RING').frame  # SpiceRef from frame kernel (fk/juice_sci_v11.tf)
'JUICE_JUP_AMALTHEA_RING'

See docs.

Fix

• Change of ESA SPICE bitbucket url API endpoint
• Upgrade pytest-cov test dependency.

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Closes #63 (closed) and #61 (closed).

changed milestone to %Version 1.0

enabled an automatic merge when the pipeline for fd180b46 succeeds

aborted the automatic merge because source branch was updated

added 2 commits

• f452a733 - Upgrade pytest-cov dependency
• 9117b1a2 - Change of ESA SPICE bitbucket url API endpoint

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changed the description

enabled an automatic merge when the pipeline for 9117b1a2 succeeds

canceled the automatic merge

added 1 commit

• 3e5c258f - Fix new URL in docstrings

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enabled an automatic merge when the pipeline for 3e5c258f succeeds

merged

mentioned in commit 435b0d8a