Add ability to use expressions in multipanelplot x and y labels

[stefaneng]

Replace check that forces use of string with more general check that allows for expressions in x/ylab.labels.

Description

Allows expressions in xlab.label and ylab.label in create.multipanelplot

Closes #90

Checklist

• This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
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• I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.

Example

Previous behavior

library(BoutrosLab.plotting.general)

p1 <- create.histogram(rnorm(100))
create.multipanelplot(
    plot.objects = list(p1),
    ylab.label = expression(bold('TEST' ~ x^2)),
    xlab.label = expression(bold('TEST' ~ y^2))
    )
#> Error in ylab.label != "": comparison is not allowed for expressions

^{Created on 2023-02-15 with reprex v2.0.2}

New behavior

library(BoutrosLab.plotting.general)

p1 <- create.histogram(rnorm(100))
create.multipanelplot(
    plot.objects = list(p1),
    ylab.label = expression(bold('TEST' ~ x^2)),
    xlab.label = expression(bold('TEST' ~ y^2))
    )

^{Created on 2023-02-15 with reprex v2.0.2}