Simplify merge method by using list and dictionary comprehensions instead of
nested for loops. This also avoids the need to check if some of the elements
of the reports arguments are None, and the need to check if reports contains
more than one report. This should also be a small performance improvement.
This change also adds a bit more detail to the docstring for the merge method.
The creation of a new empty report is also modified to no longer use an
intermediary list of keys.

This reverts commit d131657.

This reverts commit 84d7189.

If one of the values in the reports argument that merge() takes is a None,
this is now properly handled by being dropped.

Returns merge method to its original state with two nested for loops, but without
the checks for the number of dictionaries in the reports argument, or the check
that each element in reports is a dictionary. Reports are only created in the build
method so a test was made to ensure that build always returns a dictionary instead.

Previous commits I made did not import the PVFactorsReportBuilder
class correctly.

This reverts commit eeb8053.

Add bullet point about added tests for bifacial.py methods.

This reverts commit 74c8eba.

This reverts commit 4447e6c.

This reverts commit 93d0559.

This reverts commit c969994.

This reverts commit 55d8f08.