WCPS - fix FROM clause for coverage loops in aliases

[Bang Pham Huu]

For example, this WCPS query

for c in (test_mr1, test_mr2),
    d in (test_mr3)
return c + d

should generate 2 rasql queries:

 (c + d) from test_mr1 as  c, test_mr3 as d
 (c + d) from test_mr2 as  c, test_mr3 as d

also checks a WCPS result with where clause, according to ​https://doc.rasdaman.org/11_cheatsheets.html#wcps

For each iteration, the return clause is evaluated if the result of the where clause is true. Coverage iterators and alias variables can be freely used in where / return expressions.

for c in (test_mr1, test_mr2)
where some(c) > 30 return c

with the first iteration on test_mr1 with where some(c) > 30 returns false, and the second iterator where on test_mr2 returns true, then only the rasql query from the second iteration returned from petascope.