Replace backticks (``) with $() for command substitution

[anbj]

Replace the legacy syntax of backticks (``) with the preferred $() for command substitution.

[PaulWessel]

I had to learn about the legacy/obsoleteness of backticks first here. I suppose this is progress. Would you agree, @GenericMappingTools/core ?

[anbj]

See also https://mywiki.wooledge.org/BashFAQ/082.

[seisman]

Looks good to me if all the tests still pass, but csh may not support $() sytnax.

[PaulWessel]

Only csh script we have is gmt_aliases.csh and it has
set gmt_modules = (gmt --show-classic)
which fails if we do

set gmt_modules = $(gmt --show-classic)
Illegal variable name.

So we can leave that as is. No other csh syntax is shown or used anywhere. We do not want to encourage csh use anyway.

[PaulWessel]

Hi @anbj, did you run all the tests to see if there were any new failures?

[anbj]

No, sorry - not yet. I can run the tests tomorrow. I can also revert the changes for gmt_aliases.csh.

[seisman]

I just ran all tests locally. There are 4 failing tests:

	146 - anim01/anim01.sh (Failed)
	147 - anim02/anim02.sh (Failed)
	151 - anim06/anim06.sh (Failed)
	287 - gmtregress/draper.sh (Failed)

[anbj]

Great @seisman , thanks for catching these!

[seisman]

Looks good to me.

[PaulWessel]

Thanks for this work @anbj !