Heasarc: trying votable

[volodymyrss]

in response to #2333 and #2349, this is an attempt to use VOTable backend of heasarc to resolve problems with unit conversion. It needs to be determined if this is a viable approach.

[pep8speaks]

Hello @volodymyrss! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file astroquery/heasarc/tests/test_heasarc_remote.py:

Line 110:9: E265 block comment should start with '# ' Line 113:36: E225 missing whitespace around operator Line 114:35: W291 trailing whitespace Line 117:9: E265 block comment should start with '# ' Line 117:9: E303 too many blank lines (2) Line 119:27: E261 at least two spaces before inline comment Line 120:96: E251 unexpected spaces around keyword / parameter equals Line 120:98: E251 unexpected spaces around keyword / parameter equals Line 120:111: E251 unexpected spaces around keyword / parameter equals Line 120:113: E251 unexpected spaces around keyword / parameter equals Line 122:9: E265 block comment should start with '# ' Line 122:60: W291 trailing whitespace Line 123:58: E251 unexpected spaces around keyword / parameter equals Line 123:60: E251 unexpected spaces around keyword / parameter equals